cmd/docker: setFlagErrorFunc: don't load plugins for invalid flags

[thaJeztah]

relates to:

• Include plugins in docker --badopt help output #1850
  • calling docker --badopt doesn't show cli plugins #1813
• cli: FlagErrorFunc: don't print long usage output for invalid flags #5233

cmd/docker: setFlagErrorFunc: don't load plugins for invalid flags

On Docker CLI versions before v28.0.0, using an unknown flag would print
the usage output, showing all available top-level flags and commands;

docker --badopt
unknown flag: --badopt
See 'docker --help'.

Usage:  docker [OPTIONS] COMMAND

A self-sufficient runtime for containers

Options:
      --config string      Location of client config files (default "/root/.docker")
...

This output did not include plugin-commands, making the usage output
incomplete. That issue was fixed in cli@40a6cf7, which loaded all
available cli-plugins, so that a stub was created for printing the
plugin commands in the usage output. Similarly, cli@79a75da added
code to hide experimental commands and commands not supported by the
daemon.

However, since 28.0.0 (commit cli@f28fc7f), the usage output was
removed for this error, so loading plugins is no longer needed;

docker --badopt
unknown flag: --badopt

Usage:  docker [OPTIONS] COMMAND [ARG...]

Run 'docker --help' for more information

This patch removes the code added in cli@40a6cf7 and cli@79a75da.

With this patch, the output is still the same;

docker --unknown-flag buildx ls --no-such
unknown flag: --unknown-flag

Usage:  docker [OPTIONS] COMMAND [ARG...]

Run 'docker --help' for more information

This function only handles flags defined by the CLI itself; invalid
flags for plugins are handled by the plugin itself, so are not
impacted;

docker buildx ls --no-such
unknown flag: --no-such

Usage:  docker buildx ls

Run 'docker buildx ls --help' for more information

- What I did

- How I did it

- How to verify it

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/docker/docker.go	0.00%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[thaJeztah]

LOL; and all of a sudden, the linter starts to flag unrelated code;

70.72 cmd/docker/docker.go:560:74: hideUnsupportedFeatures - result 0 (error) is always nil (unparam)
70.72 func hideUnsupportedFeatures(cmd *cobra.Command, details versionDetails) error {
70.72                                                                          ^
70.72 1 issues: