cmd/docker: fail on unknown command with --version

[GitNimay]

• Fixes: docker doesn't error out on docker missing-subcommand --version  #3833
• Related to: CLI returns "unknown flag" error when you misspell a command and pass a flag #4550

Summary

docker nonexistent --version printed the CLI version and exited 0, so it couldn't be used to check whether a command or plugin (for example docker compose) is installed. Without --version the same invocation fails with unknown command.

When no built-in command or plugin matches, runDocker falls through to executing the root command. Cobra handles --version before the root command's RunE reports the unknown command. This PR unsets the root command's Version in that fallback, so the standard unknown-command error is returned. Plugin hooks still run, since the normal execution path is unchanged.

Before:

$ docker nonexistent --version
Docker version 29.x, build xxxxxxx
$ echo $?
0

After:

$ docker nonexistent --version
docker: unknown command: docker nonexistent

Run 'docker --help' for more information
$ echo $?
1

Unchanged:

• docker --version / docker -v still print the version, including when --version is passed as a global flag before an unknown argument (docker --version foo, covered by TestCliPluginsVersion).
• docker nonexistent --help still prints the top-level help (covered by TestNonexistingHelp).
• Installed plugins still handle their own --version (e.g. docker compose --version).

Added TestNonexistingVersion to e2e/cli-plugins, covering both --version and -v.

Release notes (optional)

Fix `docker <unknown-command> --version` printing the CLI version instead of an "unknown command" error.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[vvoland]

Please squash into one commit

[GitNimay]

@vvoland Done, squashed into one commit (e08aa72). The content is unchanged.

[vvoland]

Thanks!