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Docker

Commands

sudo systemctl start docker
usermod -aG docker john     # to run docker as non-root user
docker info
docker run hello-world
docker run docker/whalesay cowsay boo
docker images                   # image list
docker rmi {image}              # image remove
docker build -t docker-whale .  # build image called docker-whale
docker run -it busybox sh       # interactive shell (-i = interact, -t = pty)
docker ps                       # currently running containers
docker ps -a                    # including those which have stopped
docker ps --no-trunc            # don't truncate output
docker ps --format '{{.ID}} {{.Command}}'   # only show certain fields
docker kill {id}
docker exec -it {id} bash       # attach command in running container
docker inspect {image}          # details
docker port {container}

docker run -it --entrypoint /bin/bash --user root nginx   # override entrypoint and user
docker run -it -d -p 9090:80 -t nginx   # to connect to 9090 on local machine
docker run -d -p 9090:80 -t nginx   # to connect to 9090 on local machine

docker run -v $(pwd):/mnt -it ubuntu bash   # map/mount dir into container

docker rm $(docker ps -a -f status=exited -q)     # tidy exited containers
docker rmi $(docker images -a -q)                 # tidy (does not delete in-use images)
                                                  # above will tidy overlay dirs too
docker rmi $(docker images -f dangling=true -q)   # tidy dangling images

docker inspect --format='{{.Id}} {{.Parent}}' \
 $(docker images --filter since=$id -q)          # list child containers
docker build --no-cache .


docker login {some-registry} -u {user} -p {password}
image=my_image
docker build -t $image .
docker tag newimage {my_registry}/{some_path}/${image}:latest
docker push {my_registry}/{some_path}/${image}:latest

docker cp <containerId>:/file/path/within/container /host/path/target

Docker Compose

In same dir as docker-compose.yml

docker-compose ps

Discovering docker networking

docker network ls               # list nets
docker run --net=host ...       # share host interfaces
docker run --net=bridge ...     # container gets it's own interface and ip
docker network inspect bridge   # shows containers and ips
brctl show
brctl showmacs docker0   # is local yes = mac is on interface on docker host
                        # is local no  = mac learnt from connected container
grep . /sys/class/net/docker0/brif/veth*/port_no

Commit

exit container
docker ps -a                    # get {id} of exited container
docker commit {id} my/myname    # commit your changes to new image
docker run -it me/myname        # carry on where you left off

Notes

  • container ids abbreviate to anything unique
  • container primary process pid=1
  • container exits when primary process dies

Dockerfile

CMD

  • behaviour depends on whether ENTRYPOINT is set:
    • without ENTRYPOINT, specifies default command (and args) for docker run
    • with ENTRYPOINT, specifies default args to be added to ENTRYPOINT
  • overridden when docker run is given a command
  • string syntax gets prepended with sh -c
  • use array syntax to be precise
  • should only be one; if there are multiple, the last one wins

ENTRYPOINT

  • base command which precedes args specified by CMD
  • override with docker run --entrypoint

WORKDIR

  • set dir for next RUN, CMD or ENTRYPOINT
  • can be used multiple times override with -w

ENV

  • set environment variables
  • e.g. ENV VAR1 value
  • persists into containers created from image
  • see also -e flag

USER

  • specify user the image runs as
  • see also -u flag
  • default is root

VOLUME

  • add volumes to container
  • changes are made directly
  • not affected when you update an image
  • persist until no containers use them
  • share data between containers
  • e.g. source code, database

ADD

  • copies files into container with tar & http support
  • trailing / on target specifies source is a directory

COPY

  • like add but no support for tar or http

ONBUILD

  • execute trigger when this image is used as base for of another

ARG

defines a variable for user can to pass with build with optional default

ARG DEBIAN_FRONTEND=noninteractive

Filesystem types

Device mapper

  • used by default when AUFS is not available (e.g. on Centos 7)

AUFS

  • AUFS CoW works at file level => small changes to large files = slower
  • search order through layer = top to bottom => slower for files at bottom
  • delete places "whiteout" file in top layer

Cache miss

Failure to use a cached image, because docker build thinks the next Dockerfile directive will cause the image to change.