enhancement: show when context is overridden by "DOCKER_HOST" or other options

[thaJeztah]

Description

The Docker CLI defaults to using a local daemon through a unix-socket (Linux, macOS) or named pipe (Windows), but it can be configured to use a different connection, which can be a remote machine.

There's various ways to configure the CLI; (also see DockerCli.CurrentContext);

• DOCKER_HOST (takes precedence over context)
• --host / -H (configures the connection to use)
• DOCKER_CONTEXT (configure the context, which is used to configure the connection)
• --context (configure the context, which is used to configure the connection)
• currentContext field in ~/.docker/config.json (configures the default context to configure the connection)
• In rootless-mode, other env-vars may be involved for the default socket path.

Some of these options may not be very visible; we have some warnings when performing docker context operations to inform the user that DOCKER_HOST is set (and overrides the selected context);

cli/cli/command/context/list.go

Lines 110 to 113 in 1d768f8

	if os.Getenv(client.EnvOverrideHost) != "" {
	_, _ = fmt.Fprintf(dockerCli.Err(), "Warning: %[1]s environment variable overrides the active context. "+
	"To use a context, either set the global --context flag, or unset %[1]s environment variable.\n", client.EnvOverrideHost)
	}

cli/cli/command/context/use.go

Lines 52 to 55 in 1d768f8

	if name != command.DefaultContextName && os.Getenv(client.EnvOverrideHost) != "" {
	_, _ = fmt.Fprintf(dockerCLI.Err(), "Warning: %[1]s environment variable overrides the active context. "+
	"To use %[2]q, either set the global --context flag, or unset %[1]s environment variable.\n", client.EnvOverrideHost, name)
	}

However, other places may not show this information. Most notably docker version and docker info;

Current context (in the CLI's config) is desktop-linux;

cat ~/.docker/config.json | jq .currentContext
"desktop-linux"

However, docker version shows that the default context is used;

docker version
Client:
 Version:           28.3.0-rc.1
 API version:       1.42 (downgraded from 1.51)
 Go version:        go1.24.4
 Git commit:        f53bb88
 Built:             Fri Jun 13 18:25:06 2025
 OS/Arch:           darwin/arm64
 Context:           default

And docker info does the same;

docker info | head -n 5
Client:
 Version:    28.3.0-rc.1
 Context:    default
 Debug Mode: false
 Plugins:

In the above situation, the reason was that DOCKER_HOST was set in the current shell;

env | grep DOCKER
DOCKER_HOST=ssh://swarm-test-01/var/run/docker.sock

Improvement: make active connection more visible

⚠️ This is just a draft; detailed design still needed

1. Include "Docker Host" in docker version and docker info

We can include the active host in output of docker version and docker info;

docker version
Client:
 Version:           28.3.0-rc.1
 API version:       1.42 (downgraded from 1.51)
 Go version:        go1.24.4
 Git commit:        f53bb88
 Built:             Fri Jun 13 18:25:06 2025
 OS/Arch:           darwin/arm64
 Context:           default
 Docker Host:       ssh://swarm-test-01/var/run/docker.sock

docker info | head -n 5
Client:
 Version:     28.3.0-rc.1
 Context:     default
 Docker Host: ssh://swarm-test-01/var/run/docker.sock
 Debug Mode:  false

• ☝️ alternatively, we could include this in the Server section, however, the Server itself may be serving the API on multiple "hosts" (e.g. both on a socket AND through tcp); api: add "Listeners" field to /info, to help discover how the API is exposed moby/moby#43459

2. Show what determined the active context

We can show "why" the given context is used, e.g.;

 Context:           default (context disabled because DOCKER_HOST is set)

 Context:           desktop-linux (from DOCKER_CONTEXT)

 Context:           desktop-linux (from "~/.docker/config.json`)

⚠️ the above output may be used by users in scripting (docker version | grep 'Context:');

• Generally, users should use docker context show for this, but it may be a breaking change
• When formatting, we should make this a separate part of the template so that docker version --format '{{.Client.Context}}' only shows the context name (same for `docker info --format``)

3. Show what determined the active "host" connection

Similar to the above; show where the connection string came from;

 Docker Host:       ssh://swarm-test-01/var/run/docker.sock (from DOCKER_HOST)

❓ should we also show if this comes from the active context? (when not overridden by DOCKER_HOST)? Or is that just "noise"?

 Docker Host:       ssh://swarm-test-01/var/run/docker.sock (from "desktop-linux" context)

or

 Docker Host:       ssh://swarm-test-01/var/run/docker.sock (from current context)