A minimal docker baseimage to ease creation of X graphical application containers

This is a docker baseimage that can be used to create containers able to run any X application on a headless server very easily. The application's GUI is accessed through a modern web browser (no installation or configuration needed on the client side) or via any VNC client.

Table of Content

• Images
  • Versioning
  • Content
• Getting started
• Using the Baseimage
  • Selecting a Baseimage
  • Container Startup Sequence
  • Container Shutdown Sequence
  • Environment Variables
    • Public Environment Variables
    • Internal Environment Variables
    • Adding/Removing Internal Environment Variables
    • Availability
    • Docker Secrets
  • Ports
  • User/Group IDs
  • Locales
  • Accessing the GUI
  • Security
    • SSVNC
    • Certificates
    • VNC Password
    • DH Parameters
    • Web Authentication
      • Configuring Users Credentials
  • Initialization Scripts
  • Finalization Scripts
  • Services
    • Service Group
    • Default Service
    • Service Readiness
  • Configuration Directory
    • Application's Data Directories
  • Container Log
  • Logrotate
  • Log Monitor
    • Notification Definition
    • Notification Backend
  • Web Audio
  • Helpers
    • Adding/Removing Packages
    • Modifying Files With Sed
    • Evaluating Boolean Value
    • Taking Ownership of a Directory
    • Setting Interval Environment Variable
  • Application Icon
  • Dark Mode
    • GTK
    • QT
  • Tips and Best Practices
    • Do Not Modify Baseimage Content
    • Default Configuration Files
    • The $HOME Variable
    • Referencing Linux User/Group
    • Using rootfs Directory
    • Maximizing Only the Main Window
    • Adaptations from the 3.x Version

Images

Multiple docker images, based on different Linux distributions, are available:

Linux Distribution	Docker Image Tag	Size
Alpine 3.16	alpine-3.16-vX.Y.Z
Alpine 3.17	alpine-3.17-vX.Y.Z
Alpine 3.18	alpine-3.18-vX.Y.Z
Alpine 3.19	alpine-3.19-vX.Y.Z
Alpine 3.20	alpine-3.20-vX.Y.Z
Debian 10	debian-10-vX.Y.Z
Debian 11	debian-11-vX.Y.Z
Debian 12	debian-12-vX.Y.Z
Ubuntu 16.04 LTS	ubuntu-16.04-vX.Y.Z
Ubuntu 18.04 LTS	ubuntu-18.04-vX.Y.Z
Ubuntu 20.04 LTS	ubuntu-20.04-vX.Y.Z
Ubuntu 22.04 LTS	ubuntu-22.04-vX.Y.Z
Ubuntu 24.04 LTS	ubuntu-24.04-vX.Y.Z

Each Docker image is tagged with the Linux distribution and the release version. All release versions can be found under the Releases page.

Version part of the tag can be decomposed in the following way:

Tag	Description
distro-vX.Y.Z	Exact version of the image.
distro-vX.Y	Latest version of a specific minor version of the image.
distro-vX	Latest version of a specific major version of the image.

Finally, all available Docker image tags can also be consulted on Docker Hub.

Versioning

Images are versioned. Version number follows the semantic versioning. The version format is MAJOR.MINOR.PATCH, where an increment of the:

• MAJOR version indicates that a backwards-incompatible change has been done.
• MINOR version indicates that functionality has been added in a backwards-compatible manner.
• PATCH version indicates that a bug fix has been done in a backwards-compatible manner.

Content

Here are the main components of the baseimage:

• An init system.
• A process supervisor, with proper PID 1 functionality (proper reaping of processes).
• TigerVNC, a X server with an integrated VNC server.
• Openbox, a window manager.
• noVNC, a HTML5 VNC client.
• NGINX, a high-performance HTTP server.
• Useful tools to ease container building.
• Environment to better support dockerized applications.

Getting started

The Dockerfile for your application can be very simple, as only three things are required:

• Instructions to install the application.
• A script that starts the application (stored at /startapp.sh in container).
• The name of the application.

Here is an example of a docker file that would be used to run the xterm terminal.

In Dockerfile:

# Pull base image.
FROM jlesage/baseimage-gui:alpine-3.19-v4

# Install xterm.
RUN add-pkg xterm

# Copy the start script.
COPY startapp.sh /startapp.sh

# Set the name of the application.
RUN set-cont-env APP_NAME "Xterm"

In startapp.sh:

#!/bin/sh
exec /usr/bin/xterm

Make sure the file is executable, by running chmod +x startapp.sh.

Then, build your docker image:

docker build -t docker-xterm .

And run it:

docker run --rm -p 5800:5800 -p 5900:5900 docker-xterm

You should be able to access the xterm GUI by opening in a web browser:

http://[HOST IP ADDR]:5800

Using the Baseimage

Selecting a Baseimage

Using a baseimage based on Alpine Linux is the recommended choice. Not only because of its small size, but also because Alpine Linux is a distribution based on musl and BusyBox that is designed for security, simplicity and resource efficiency.

However, using this baseimage to integrate an application not part of the Alpine's software repository or without its source code available may be harder. This is because Alpine Linux uses musl C standard library instead of GNU C library (glibc) that most applications are built against. Compatibility between these two libraries is very limited.

Else, Debian and Ubuntu images are well known Linux distributions that provide great compatibility with existing applications.

Container Startup Sequence

When the container is starting, the following steps are performed:

• The init process (/init) is invoked.
• Internal environment variables are loaded from /etc/cont-env.d.
• Initialization scripts under /etc/cont-init.d are executed in alphabetical order.
• Control is given to the process supervisor.
• The service group /etc/services.d/default is loaded, along with its dependencies.
• Services are started, in proper order.
• Container is now fully started.

Container Shutdown Sequence

There are two ways a container can shutdown:

1. When the implemented application terminates.
2. When Docker performs a shutdown of the container (e.g via the docker stop command).

In both cases, the shutdown sequence is:

• All services are terminated, in reverse order.
• If some processes are still alive, a SIGTERM is sent to everyone.
• After 5 seconds, all remaining processes are forcefully terminated via the SIGKILL signal.
• The process supervisor execute the exit script (/etc/services.d/exit).
• The exit script executes, in alphabetical order, finalization scripts defined under /etc/cont-finish.d/.
• Container is full stopped.

Environment Variables

Environment variables are very useful to customize the behavior of the container and its application.

There are two types of environment variables:

• Public: These variables are targeted to people using the container. They provide a way to configure it. They are declared in the Dockerfile, via the ENV instruction. Their value can be set by users during the creation of the container, via the -e "<VAR>=<VALUE>" argument of the docker run command. Also, many Docker container management systems use these variables to automatically provide configuration parameters to the user.

• Internal: These variables are the ones that don't need to be exposed to users. They are useful for the application itself, but are not intended to be changed by users.

NOTE: If a variable is defined as both an internal and public one, the value of the public variable takes precedence.

Public Environment Variables

The following public environment variables are provided by the baseimage:

Variable	Description	Default
USER_ID	ID of the user the application runs as. See User/Group IDs to better understand when this should be set.	1000
GROUP_ID	ID of the group the application runs as. See User/Group IDs to better understand when this should be set.	1000
SUP_GROUP_IDS	Comma-separated list of supplementary group IDs of the application.	(no value)
UMASK	Mask that controls how permissions are set for newly created files and folders. The value of the mask is in octal notation. By default, the default umask value is 0022, meaning that newly created files and folders are readable by everyone, but only writable by the owner. See the online umask calculator at http://wintelguy.com/umask-calc.pl.	0022
LANG	Set the locale, which defines the application's language, if supported. Format of the locale is language[_territory][.codeset], where language is an ISO 639 language code, territory is an ISO 3166 country code and codeset is a character set, like UTF-8. For example, Australian English using the UTF-8 encoding is en_AU.UTF-8.	en_US.UTF-8
TZ	TimeZone used by the container. Timezone can also be set by mapping /etc/localtime between the host and the container.	Etc/UTC
KEEP_APP_RUNNING	When set to 1, the application will be automatically restarted when it crashes or terminates.	0
APP_NICENESS	Priority at which the application should run. A niceness value of -20 is the highest priority and 19 is the lowest priority. The default niceness value is 0. NOTE: A negative niceness (priority increase) requires additional permissions. In this case, the container should be run with the docker option --cap-add=SYS_NICE.	0
INSTALL_PACKAGES	Space-separated list of packages to install during the startup of the container. Packages are installed from the repository of the Linux distribution this container is based on. ATTENTION: Container functionality can be affected when installing a package that overrides existing container files (e.g. binaries).	(no value)
PACKAGES_MIRROR	Mirror of the repository to use when installing packages.	(no value)
CONTAINER_DEBUG	Set to 1 to enable debug logging.	0
DISPLAY_WIDTH	Width (in pixels) of the application's window.	1920
DISPLAY_HEIGHT	Height (in pixels) of the application's window.	1080
DARK_MODE	When set to 1, dark mode is enabled for the application.	0
WEB_AUDIO	When set to 1, audio support is enabled, meaning that any audio produced by the application is played through the browser. Note that audio is not supported for VNC clients.	0
WEB_AUTHENTICATION	When set to 1, the application' GUI is protected via a login page when accessed via a web browser. Access is allowed only when providing valid credentials. NOTE: This feature requires secure connection (SECURE_CONNECTION environment variable) to be enabled.	0
WEB_AUTHENTICATION_USERNAME	Optional username to configure for the web authentication. This is a quick and easy way to configure credentials for a single user. To configure credentials in a more secure way, or to add more users, see the Web Authentication section.	(no value)
WEB_AUTHENTICATION_PASSWORD	Optional password to configure for the web authentication. This is a quick and easy way to configure credentials for a single user. To configure credentials in a more secure way, or to add more users, see the Web Authentication section.	(no value)
SECURE_CONNECTION	When set to 1, an encrypted connection is used to access the application's GUI (either via a web browser or VNC client). See the Security section for more details.	0
SECURE_CONNECTION_VNC_METHOD	Method used to perform the secure VNC connection. Possible values are SSL or TLS. See the Security section for more details.	SSL
SECURE_CONNECTION_CERTS_CHECK_INTERVAL	Interval, in seconds, at which the system verifies if web or VNC certificates have changed. When a change is detected, the affected services are automatically restarted. A value of 0 disables the check.	60
WEB_LISTENING_PORT	Port used by the web server to serve the UI of the application. This port is used internally by the container and it is usually not required to be changed. By default, a container is created with the default bridge network, meaning that, to be accessible, each internal container port must be mapped to an external port (using the -p or --publish argument). However, if the container is created with another network type, changing the port used by the container might be useful to prevent conflict with other services/containers. NOTE: a value of -1 disables listening, meaning that the application's UI won't be accessible over HTTP/HTTPs.	5800
VNC_LISTENING_PORT	Port used by the VNC server to serve the UI of the application. This port is used internally by the container and it is usually not required to be changed. By default, a container is created with the default bridge network, meaning that, to be accessible, each internal container port must be mapped to an external port (using the -p or --publish argument). However, if the container is created with another network type, changing the port used by the container might be useful to prevent conflict with other services/containers. NOTE: a value of -1 disables listening, meaning that the application's UI won't be accessible over VNC.	5900
VNC_PASSWORD	Password needed to connect to the application's GUI. See the VNC Password section for more details.	(no value)
ENABLE_CJK_FONT	When set to 1, open-source computer font WenQuanYi Zen Hei is installed. This font contains a large range of Chinese/Japanese/Korean characters.	0

Internal Environment Variables

The following internal environment variables are provided by the baseimage:

Variable	Description	Default
APP_NAME	Name of the implemented application.	DockerApp
APP_VERSION	Version of the implemented application.	(no value)
DOCKER_IMAGE_VERSION	Version of the Docker image that implements the application.	(no value)
DOCKER_IMAGE_PLATFORM	Platform (OS / CPU architecture) of the Docker image that implements the application.	(no value)
HOME	Home directory.	(no value)
XDG_CONFIG_HOME	Defines the base directory relative to which user specific configuration files should be stored.	/config/xdg/config
XDG_DATA_HOME	Defines the base directory relative to which user specific data files should be stored.	/config/xdg/data
XDG_CACHE_HOME	Defines the base directory relative to which user specific non-essential data files should be stored.	/config/xdg/cache
TAKE_CONFIG_OWNERSHIP	When set to 0, ownership of the content of the /config directory is not taken during startup of the container.	1
INSTALL_PACKAGES_INTERNAL	Space-separated list of packages to install during the startup of the container. Packages are installed from the repository of the Linux distribution this container is based on.	(no value)
SUP_GROUP_IDS_INTERNAL	Comma-separated list of supplementary group IDs of the application. These are merged with the ones that might be supplied by SUP_GROUP_IDS.	(no value)
SERVICES_GRACETIME	During container shutdown, this defines the amount of time (in milliseconds) allowed to services to gracefully terminate before sending the KILL signal to everyone.	5000

Adding/Removing Internal Environment Variables

Internal environment variables are defined by adding a file to /etc/cont-env.d/ inside the container, where the name of the file is the name of the variable and its value is defined by the content of the file.

If the file has execute permission, the init process will execute the program and the value of the environment variable is expected to be printed to its standard output.

NOTE: If the program exits with the return code 100, the environment variable is not set (this is different than being set with an empty value).

NOTE: Any output to stderr performed by the program is redirected to the container's log.

NOTE: The helper set-cont-env can be used to set internal environment variables from the Dockerfile.

Availability

Since public environment variables are defined during the creation of the container, they are always available to all your scripts and services, as soon as the container starts.

For internal environment variables, they first need to be loaded during the startup of the container before they can be used. Since this is done before running init scripts and services, availability should not be an issue.

Docker Secrets

Docker secrets is a functionality available to swarm services that offers a secure way to store sensitive information such as username, passwords, etc.

This baseimage automatically exports, as environment variables, Docker secrets that follow this naming convention:

CONT_ENV_<environment variable name>

For example, for a secret named CONT_ENV_MY_PASSWORD, the environment variable MY_PASSWORD is created, with its content matching the one of the secret.

Ports

Here is the list of ports used by the baseimage. With a container using the default bridge network, these ports can be mapped to the host via the -p <HOST_PORT>:<CONTAINER_PORT> parameter.

Port	Mapping to host	Description
5800	Optional	Port to access the application's GUI via the web interface. Mapping to the host is optional if access through the web interface is not wanted. For a container not using the default bridge network, the port can be changed with the WEB_LISTENING_PORT environment variable.
5900	Optional	Port to access the application's GUI via the VNC protocol. Mapping to the host is optional if access through the VNC protocol is not wanted. For a container not using the default bridge network, the port can be changed with the VNC_LISTENING_PORT environment variable.

User/Group IDs

When mapping data volumes (via the -v flag of the docker run command), permissions issues can occur between the host and the container. Files and folders of a data volume are owned by a user, which is probably not the same as the default user under which the implemented application is running. Depending on permissions, this situation could prevent the container from accessing files and folders on the shared volume.

To avoid this problem, you can specify the user the application should run as.

This is done by passing the user ID and group ID to the container via the USER_ID and GROUP_ID environment variables.

To find the right IDs to use, issue the following command on the host, with the user owning the data volume on the host:

id <username>

Which gives an output like this one:

uid=1000(myuser) gid=1000(myuser) groups=1000(myuser),4(adm),24(cdrom),27(sudo),46(plugdev),113(lpadmin)

The value of uid (user ID) and gid (group ID) are the ones that you should be given the container.

Locales

The default locale of the container is set to POSIX. If this cause issues with your application, the proper locale can be installed. For example, adding the following instructions to your Dockerfile set the locale to en_US.UTF-8.

RUN \
    add-pkg locales && \
    sed-patch 's/# en_US.UTF-8 UTF-8/en_US.UTF-8 UTF-8/' /etc/locale.gen && \
    locale-gen
ENV LANG=en_US.UTF-8

NOTE: Locales are not supported by musl C standard library on Alpine. See:

• http://wiki.musl-libc.org/wiki/Open_Issues#C_locale_conformance
• gliderlabs/docker-alpine#144

Accessing the GUI

Assuming that container's ports are mapped to the same host's ports, the graphical interface of the application can be accessed via:

• A web browser:

http://<HOST IP ADDR>:5800

• Any VNC client:

<HOST IP ADDR>:5900

Security

By default, access to the application's GUI is done over an unencrypted connection (HTTP or VNC).

Secure connection can be enabled via the SECURE_CONNECTION environment variable. See the Environment Variables section for more details on how to set an environment variable.

When enabled, application's GUI is performed over an HTTPs connection when accessed with a browser. All HTTP accesses are automatically redirected to HTTPs.

When using a VNC client, the VNC connection is performed over SSL. Note that few VNC clients support this method. SSVNC is one of them.

SSVNC

SSVNC is a VNC viewer that adds encryption security to VNC connections.

While the Linux version of SSVNC works well, the Windows version has some issues. At the time of writing, the latest version 1.0.30 is not functional, as a connection fails with the following error:

ReadExact: Socket error while reading

However, for your convienence, an unoffical and working version is provided here:

https://github.com/jlesage/docker-baseimage-gui/raw/master/tools/ssvnc_windows_only-1.0.30-r1.zip

The only difference with the offical package is that the bundled version of stunnel has been upgraded to version 5.49, which fixes the connection problems.

Certificates

Here are the certificate files needed by the container. By default, when they are missing, self-signed certificates are generated and used. All files are PEM encoded, x509 certificates.

Container Path	Purpose	Content
/config/certs/vnc-server.pem	VNC connection encryption.	VNC server's private key and certificate, bundled with any root and intermediate certificates.
/config/certs/web-privkey.pem	HTTPs connection encryption.	Web server's private key.
/config/certs/web-fullchain.pem	HTTPs connection encryption.	Web server's certificate, bundled with any root and intermediate certificates.

NOTE: To prevent any certificate validity warnings/errors from the browser or VNC client, make sure to supply your own valid certificates.

NOTE: Certificate files are monitored and relevant daemons are automatically restarted when changes are detected.

VNC Password

To restrict access to your application, a password can be specified. This can be done via two methods:

• By using the VNC_PASSWORD environment variable.
• By creating a .vncpass_clear file at the root of the /config volume. This file should contains the password in clear-text. During the container startup, content of the file is obfuscated and moved to .vncpass.

The level of security provided by the VNC password depends on two things:

• The type of communication channel (encrypted/unencrypted).
• How secure access to the host is.

When using a VNC password, it is highly desirable to enable the secure connection to prevent sending the password in clear over an unencrypted channel.

Access to the host by unexpected users with sufficient privileges can be dangerous as they can retrieve the password with the following methods:

• By looking at the VNC_PASSWORD environment variable value via the docker inspect command. By defaut, the docker command can be run only by the root user. However, it is possible to configure the system to allow the docker command to be run by any users part of a specific group.
• By decrypting the /config/.vncpass file. This requires the user to have the appropriate permission to read the file: it has to be root or be the user defined by the USER_ID environment variable.

DH Parameters

Diffie-Hellman (DH) parameters define how the DH key-exchange is performed. More details about this algorithm can be found on the OpenSSL Wiki.

DH Parameters are saved into the PEM encoded file located inside the container at /config/certs/dhparam.pem. By default, when this file is missing, 2048 bits DH parameters are automatically generated. Note that this one-time operation takes some time to perform and increases the startup time of the container.

Web Authentication

Access to the application's GUI via a web browser can be protected with a login page. When web authentication is enabled, users have to provide valid credentials, otherwise access is denied.

Web authentication can be enabled by setting the WEB_AUTHENTICATION environment variable to 1.

See the Environment Variables section for more details on how to set an environment variable.

NOTE: Secure connection must be also enabled to use web authentication. See the Security section for more details.

Configuring Users Credentials

Two methods can be used to configure users credentials:

1. Via container environment variables.
2. Via password database.

Containers environment variables can be used to quickly and easily configure a single user. Username and pasword are defined via the following environment variables:

• WEB_AUTHENTICATION_USERNAME
• WEB_AUTHENTICATION_PASSWORD

See the Environment Variables section for more details on how to set an environment variable.

The second method is more secure and allows multiple users to be configured. The usernames and password hashes are saved into a password database, located at /config/webauth-htpasswd inside the container. This database file has the same format as htpasswd files of the Apache HTTP server. Note that password themselves are not saved into the database, but only their hash. The bcrypt password hashing function is used to generate hashes.

Users are managed via the webauth-user tool included in the container:

• To add a user password: docker exec -ti <container name or id> webauth-user add <username>.
• To update a user password: docker exec -ti <container name or id> webauth-user update <username>.
• To remove a user: docker exec <container name or id> webauth-user del <username>.
• To list users: docker exec <container name or id> webauth-user user.

Initialization Scripts

During the container startup, initialization scripts are executed in alphabetical order. They are executed before starting services.

Initialization scripts are located at /etc/cont-init.d/ inside the container.

To have a better predictability of the execution order, name of the scripts follows the XX-name.sh format, where XX is a sequence number.

The following ranges are used by the baseimage:

• 10-29
• 70-89

Unless specific needs are required, containers built against this baseimage should use the range 50-59.

Finalization Scripts

Finalization scripts are executed, in alphabetical order, during the shutdown process of the container. They are executed after all services have been stopped.

Finalization scripts are located under /etc/cont-finish.d/ inside the container.

Services

Services are programs handled by the process supervisor that run in background. When a service dies, it can be configured to be automatically restarted.

Services are defined under /etc/services.d/ in the container. Each service has its own directory, in which different files are used to store the behavior of the service.

The content of files provides the value for the associated configuration setting. If the file has execution permission, it will be executed by the process supervisor and its output is taked as the value of the configuration setting.

File	Type	Description	Default
run	Program	The program to run.	N/A
is_ready	Program	Program invoked by the process supervisor to verify if the service is ready. The program should exit with an exit code of 0 when service is ready. PID of the service if given to the program as parameter.	N/A
kill	Program	Program to run when service needs to be killed. The PID of the service if given to the program as parameter. Note that the TERM signal is still sent to the service after executing the program.	N/A
finish	Program	Program invoked when the service terminates. The service's exit code is given to the program as parameter.	N/A
params	String	Parameter for the service's program to run. One parameter per line.	No parameter
environment	String	Environment to use for the service. One environment variable per line, of the form key=value.	Environment untouched
environment_extra	String	Extra variables to add to the environment of the service. One environment variable per line, of the form key=value.	No extra variable
respawn	Boolean	Whether or not the process must be respawned when it dies.	FALSE
sync	Boolean	Whether or not the process supervisor waits until the service ends. This is mutually exclusive with respawn.	FALSE
ready_timeout	Unsigned integer	Maximum amount of time (in milliseconds) to wait for the service to be ready.	5000
interval	Interval	Interval, in seconds, at which the service should be executed. This is mutually exclusive with respawn.	No interval
uid	Unsigned integer	The user ID under which the service will run.	$USER_ID
gid	Unsigned integer	The group ID under which the service will run.	$GROUP_ID
sgid	Unsigned integer	List of supplementary group IDs of the service. One group ID per line.	Empty list
umask	Octal integer	The umask value (in octal notation) of the service.	0022
priority	Signed integer	Priority at which the service should run. A niceness value of -20 is the highest priority and 19 is the lowest priority.	0
workdir	String	The working directory of the service.	Service's directory path
ignore_failure	Boolean	When set, the inability to start the service won't prevent the container to start.	FALSE
shutdown_on_terminate	Boolean	Indicates that the container should be shut down when the service terminates.	FALSE
min_running_time	Unsigned integer	The minimum amount of time (in milliseconds) the service should be running before considering it as ready.	500
disabled	Boolean	Indicates that the service is disabled, meaning that it won't be loaded nor started.	FALSE
.dep	Boolean	Indicates that the service depends on another one. For example, having srvB.dep means that srvB should be started before this service.	N/A

The following table provides more details about some value types:

Type	Description
Program	An executable binary, a script or a symbolic link to the program to run. The program file must have the execute permission.
Boolean	A boolean value. A true value can be 1, true, on, yes, y, enable, enabled. A false value can be 0, false, off, no, n, disable, disabled. Values are case insensitive. Also, the presence of an empty file indicates a true value (i.e. the file can be "touched").
Interval	An unsigned integer value. The following values are also accepted (case insensitive): yearly, monthly, weekly, daily, hourly.

Service Group

A service group is a service for which there is no run program. The process supervisor will only load its dependencies.

Default Service

During startup, the process supervisor first load the service group default. This service group contains dependencies to services that should be started and that are not a dependency of the app service.

Service Readiness

By default, a service is considered ready once it has been successfully launched and ran for a minimum amount of time (500ms by default).

This behavior can be adjusted with the following methods:

• By adjusting the minimum amount of time the service should run before considering it as ready. This can be done by adding the min_running_time file to the service's directory.
• By informing the process supervisor when the service is ready. This is done by adding the is_ready program to the service's directory, along with ready_timeout file to indicate the maximum amount of time to wait for the service to be ready.

Configuration Directory

Applications often need to write configuration, data, states, logs, etc. Inside the container, this data should be stored under the /config directory.

This directory is intended to be mapped to a folder on the host. The goal is to write stuff outside the container to keep this data persistent.

NOTE: During the container startup, ownership of this folder and all its content is taken. This is to make sure that /config can be accessed by the user configured through USER_ID/GROUP_ID. This behavior can be adjusted via the TAKE_CONFIG_OWNERSHIP internal environment variable.

Application's Data Directories

A lot of applications use the environment variables defined by the XDG Base Directory Specification to determine where to store various data. The baseimage sets these variables so they all fall under /config/:

• XDG_DATA_HOME=/config/xdg/data
• XDG_CONFIG_HOME=/config/xdg/config
• XDG_CACHE_HOME=/config/xdg/cache
• XDG_STATE_HOME=/config/xdg/state

Container Log

Everything written to the standard output and standard error output of scripts executed by the init process and services is saved into the container's log. The container log can be viewed with the command docker logs <name of the container>.

To ease consultation of the log, all messages are prefixed with the name of the service or script. Also, it is a good idea to limit the number of information written to this log. If a program's output is too verbose, it is preferable to redirect it to a file. For example, the run command of a service that redirects the standard output and standard error output to different files could be:

#!/bin/sh
exec /usr/bin/my_service > /config/log/my_service_out.log 2> /config/log/my_service_err.log

Logrotate

The baseimage integrates logrotate, an utility used to rotate and compress log files. This tool runs automatically once a day via a service. The service is automatically disabled when no log files are configured.

To enable the rotation/compression of a log file, a configuration file needs to be added to the /etc/cont-logrotate.d directory inside the container. This configuration defines how to handle this specific log file.

Here is a simple example of a configuration defined at /etc/cont-logrotate.d/myapp:

/config/log/myapp.log {
    minsize 1M
}

This configuration file can override the default parameters, which are defined at /opt/base/etc/logrotate.conf inside the container. In summary, by default:

• Log files are rotated weekly.
• Four weeks worth of backlogs are kept.
• Rotated log files are compressed.
• Date is used as a suffix of rotated log files.

For more details about the content of logrotate configuration files, see the manual at https://linux.die.net/man/8/logrotate.

Log Monitor

The baseimage includes a simple log monitor. This monitor allows sending notification(s) when a particular message is detected in a log or status file.

This system has two main components:

• Notification definitions: Describe properties of a notification (title, message, severity, etc), how it is triggered (filtering function) and the associated monitored file(s).
• Backends (targets): Once a matching string is found in a file, a notification is triggered and sent to one or more backends. A backend can implement any functionality. For example, it could send the notification to the container's log, a file or an online service.

There are two types of files that can be monitored:

• Log files: A log file is a file having new content appended to it.
• *Status files: A status file doesn't have new content appended. Instead, its whole content is refreshed/overwritten periodically.

Notification Definition

The definition of a notification consists in multiple files, stored in a directory under /etc/logmonitor/notifications.d inside the container. For example, definition of notification MYNOTIF is found under /etc/logmonitor/notifications.d/MYNOTIF/.

The following table describe files part of the definition:

File	Mandatory	Description
filter	Yes	Program (script or binary with executable permission) used to filter messages from a log file. It is invoked by the log monitor with a single argument: a line from the log file. On a match, the program should exit with a value of 0. Any other values is interpreted as non-match.
title	Yes	File containing the title of the notification. To produce dynamic content, the file can be a program (script or binary with executable permission). In this case, the program is invoked by the log monitor with the matched message from the log file as the single argument. Output of the program is used as the notification's title.
desc	Yes	File containing the description/message of the notification. To produce dynamic content, the file can be a program (script or binary with executable permission). In this case, the program is invoked by the log monitor with the matched message from the log file as the single argument. Output of the program is used as the notification's description/message.
level	Yes	File containing severity level of the notification. Valid severity level values are ERROR, WARNING or INFO. To produce dynamic content, the file can be a program (script or binary with executable permission). In this case, the program is invoked by the log monitor with the matched message from the log file as the single argument. Output of the program is used as the notification's severity level.
source	Yes	File containing the absolute path(s) to file(s) to monitor (one path per line). Prepend the path with status: to indicate that the file is a status file. A path with prefixed with log: or without any prefix is considered as a log file.

Notification Backend

Definition of a notification backend is stored in a directory under /etc/cont-logmonitor/targets.d. For example, definition of STDOUT backend is found under /etc/cont-logmonitor/target.d/STDOUT/. The following table describe files part of the definition:

File	Mandatory	Description
send	Yes	Program (script or binary with executable permission) that sends the notification. It is invoked by the log monitor with the following notification properties as arguments: title, description/message and the severity level.
debouncing	No	File containing the minimum amount time (in seconds) that must elapse before sending the same notification with this backend. A value of 0 means infinite (notification is sent once). If this file is missing, no debouncing is done.

By default, the baseimage contains the following notification backends:

Backend	Description	Debouncing time
stdout	Display a message to the standard output, making it visible in the container's log. Message of the format is {LEVEL}: {TITLE} {MESSAGE}.	21 600s (6 hours)
yad	Display the notification in a window box, visible in the application's GUI.	Infinite

Web Audio

The baseimage supports streaming audio from any application that supports PulseAudio. The audio plays through the web browser of users. Audio is not supported when the application is accessed via a VNC client.

Audio is streamed with the following specification:

• Raw PCM format
• 2 channels
• 16 bit sample depth
• 44.1KHZ sample rate

Web audio support can be enabled by setting the value of the WEB_AUDIO environment variable to 1. See the Environment Variables section for more details on how to set an environment variable.

Once support is enabled, PulseAudio environment is automatically configured for the application and additional services are started to capture and stream the audio.

Helpers

The baseimage contains a few helpers that can be used when bulding a container or during the execution of a container.

Adding/Removing Packages

To add or remove packages, use the helpers add-pkg and del-pkg provided by this baseimage. To minimize the size of the container, these tools perform proper cleanup and make sure that no useless files are left after addition or removal of packages.

Also, these tools can be used to easily install a group packages temporarily. Using the --virtual NAME parameter, this allows installing packages and remove them at a later time using the provided NAME (no need to repeat given packages).

Note that if a specified package is already installed, it will be ignored and will not be removed automatically. For example, the following commands could be added to Dockerfile to compile a project:

RUN \
    add-pkg --virtual build-dependencies build-base cmake git && \
    git clone https://myproject.com/myproject.git
    make -C myproject && \
    make -C myproject install && \
    del-pkg build-dependencies

Supposing that, in the example above, the git package was already installed when the call to add-pkg is performed, running del-pkg build-dependencies doesn't remove it.

Modifying Files With Sed

sed is a useful tool often used in container builds to modify files. However, one downside of this method is that there is no easy way to determine if sed actually modified the file or not.

It's for this reason that the baseimage includes a helper that gives sed a "patch-like" behavior: if applying a sed expression results in no change on the target file, then an error is reported. This helper is named sed-patch and has the following usage:

sed-patch [SED_OPT]... SED_EXPRESSION FILE

Note that the sed option -i (edit files in place) is already supplied by the helper.

It can be used in Dockerfile, for example, like this:

RUN sed-patch 's/Replace this/By this/' /etc/myfile

If running this sed expression doesn't bring any change to /etc/myfiles, the command fails and thus, the Docker build also.

Evaluating Boolean Value

Environment variables are often used to store a boolean value. Using the helpers is-bool-value-true and is-bool-value-false allows to easily determine if a value is "true" or "false".

The following values are considered "true":

• 1
• true
• yes
• enabled
• enable
• on

The following values are considered "false":

• 0
• false
• no
• disabled
• disable
• off

For example, the following shell script snippet checks if the environment variable CONTAINER_DEBUG contains a "true" value:

if is-bool-value-true "${CONTAINER_DEBUG:-0}"; then
    # Do something...
fi

Taking Ownership of a Directory

The helper take-ownership recursively sets the user ID and group ID of a directory and all the files and directories under it.

This helper is well suited for scenarios where the directory is mapped to the host. If on the host this directory is a network share, setting/changing the ownership via chown can fail. The helper handles this case by ignoring the failure if a write test turns out to be positive.

For example, the following command take ownership of /config, by automatically using the user and group IDs from the USER_ID and GROUP_ID environment variables:

take-ownership /config

User and group IDs can also be explicit. For example, to set ownership to user ID 99 and group ID 100:

take-ownership /config 99 100

Setting Interval Environment Variable

The helper set-cont-env can be used to set internal environment variables from the Dockerfile.

For example, the following line can be added to the Dockerfile to set the value of the APP_NAME internal environment variable:

RUN set-cont-env APP_NAME "Xterm"

This automatically creates the environment variable file under /etc/cont-env.d.

Application Icon

A picture of your application can be added to the image. This picture is displayed in the WEB interface's navigation bar. This is also the master picture used to generate favicons that support different browsers and platforms.

Add the following command to your Dockerfile, with the proper URL pointing to your master icon: The master icon should be a square PNG image with a size of at least 260x260 for optimal results.

# Generate and install favicons.
RUN \
    APP_ICON_URL=https://github.com/jlesage/docker-templates/raw/master/jlesage/images/generic-app-icon.png && \
    install_app_icon.sh "$APP_ICON_URL"

Note that favicons are generated by RealFaviconGenerator.

Dark Mode

Dark mode can be enabled via the DARK_MODE environment variable. When enabled, the web interface used to display the application is automatically adjusted accordingly.

For the application itself, supporting dark mode is more complicated. Applications don't use the same toolkit to build their UI and each toolkit has its own way to activate the dark mode.

The baseimage provides support for the GTK and QT toolkits.

GTK

When dark mode is enabled, the baseimage automatically setups the environment to force the application to use a dark theme. Under the hood, this is done by setting the following environment variables:

• GTK_THEME is set to Adwaita:dark. This is used by GTK3 and GTK4 applications.
• GTK2_RC_FILES is set to /opt/base/share/themes/Dark/gtk-2.0/gtkrc. This is used by GTK2 applications.

QT

When dark mode is enabled, the baseimage automatically setup the environment to force the application to use a dark theme. Under the hood, this is done by setting the QT_STYLE_OVERRIDE environment variable to Adwaita-Dark.

In addition, the application's Dockerfile should install the Adwaita style/theme. It is provided by the adwaita-qt package, available from the Ubuntu, Debian or Alpine Linux software repositories.

NOTE: Dark mode is currently supported by QT5 and QT6.

Tips and Best Practices

Do Not Modify Baseimage Content

Try to avoid modifications to files provided by the baseimage. This minimizes the risk of breaking your container after using a new version of the baseimage.

Default Configuration Files

It is often useful to keep the original version of a configuration file. For example, a copy of the original file could be modified by an initialization script before being installed.

These original files, also called default files, should be stored under the /defaults directory inside the container.

The $HOME Variable

The application is run under a Linux user having its own ID. This user has no login capability, has no password, no valid login shell and no home directory. It is effectively a kind of user used by daemons.

Thus, by default, the $HOME environment variable is not set. While this should be fine in most case, some applications may expect the $HOME environment variable to be set (since normally the application is run by a logged user) and may not behave correctly otherwise.

To make the application happy, the home directory can be set at the beginning of the startapp.sh script:

export HOME=/config

Adjust the location of the home directory to fit your needs. However, if the application uses the home directory to write data, make sure it is done in a volume mapped to the host (e.g. /config),

Note that the same technique can be used by services, by exporting the home directory into their run script.

Referencing Linux User/Group

The Linux user/group under which the application is running can be referenced via:

• Its ID, as indicated by the USER_ID/GROUP_ID environment variable.
• By the user/group app. The app user/group is setup during the startup to match the configured USER_ID/GROUP_ID.

Using rootfs Directory

All files that need to be copied into the container should be stored in your source tree under the directory rootfs. The folder structure into this directory should reflect the structure inside the container. For example, the file /etc/cont-init.d/my-init.sh inside the container should be saved as rootfs/etc/cont-init.d/my-init.sh in your source tree.

This way, copying all the required files to the correct place into the container can be done with this single line in your Dockerfile:

COPY rootfs/ /

Maximizing Only the Main Window

By default, the application's window is maximized and decorations are hidden. When the application has multiple windows, this behavior may need to be restricted to only the main one.

The window manager can be configured to apply different behaviors for different windows of the application. A specific window is identified by matching one or more of its properties:

• Name of the window.
• Class of the window.
• Title of the window.
• Type of the window.
• etc.

To find the value of a property for a particular window:

• Create and start an instance of the container.
• From the host, start the obxprop tool:

docker exec <container name or id> obxprop | grep "^_OB_APP"

• Access the GUI of the application and click somewhere on the interested window.
• Information about that window will be printed.

The following table shows how to find the relevant information:

Property	Value
Name	The window's _OB_APP_NAME property.
Class	The window's _OB_APP_CLASS property.
Title	The window's _OB_APP_TITLE property.
GroupName	The window's _OB_APP_GROUP_NAME property.
GroupClass	The window's _OB_APP_GROUP_CLASS property.
Type	The window's _OB_APP_TYPE property. The type can be one of the following values: desktop, dialog, dock, menu, normal, notification, splash, toolbar, utility.
Role	The window's _OB_APP_ROLE property.

By default, the window manager configuration matches only the type of the window, which must be normal. More restrictions can be added to better select the correct window.

To do this, matching criterias can be defined by adding a file located at /etc/openbox/main-window-selection.xml in the container. This file should have one matching critera per line, in XML format. For example, to match against both the type and the name of the window, the file content should be:

<Type>normal</Type>
<Name>My Application</Name>

NOTE: To maintain backward compatibility with previous 4.x versions, the container fallbacks to /etc/jwm/main-window-selection.jwmrc if /etc/openbox/main-window-selection.xml does not exist.

Adaptations from the 3.x Version

For existing applications using the previous version of the baseimage, few adaptations are needed when updating to the new baseimage. Here are a few tips:

• Verify exposed environment variables: each of them should be categorized as a public or private one. See the Environment Variables section.
• Initialization scripts should be renamed to have the proper naming format. See the Initialization Scripts section.
• Parameters/definition of services should be adjusted for the new system. See the Services section.
• Verify that no scripts are using with-contenv in their shebang (e.g. from init scripts).
• Set the APP_VERSION and DOCKER_IMAGE_VERSION internal environment variables when/if needed.
• Any adjustment to the window manager config (e.g. to maximize only the main window) should be adapted to use the new mechanism. See the Maximizing Only the Main Window section.