docker-buildx: symlink plugin to brew owned cli-plugins

[narqo]

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The PR updates the existing formula with docker-buildx Docker plugin, adding a post install step, that links the plugin into #{HOMEBREW_PREFIX}/lib/docker/cli-plugins directory. Users are expected to add the directory to the list of cliPluginsExtraDirs in their docker-cli's config.

E.g. this is how it looks in my config.json

% cat ~/.config/docker/config.json
{
    "auths": {},
    "currentContext": "colima",
    "cliPluginsExtraDirs": [
        "/opt/homebrew/lib/docker/cli-plugins"
    ]
}

I find this behaviour much more in-line with how managed packages should work. That is, as a user I find it more natural to update my local config, to point to a managed directory with plugins, than symlinking the managed plugin into my home directory (i.e. the previous suggestion from "caveats").

---

Note on Docker's cliPluginsExtraDirs, this parameter was added to docker-cli more than 4 years ago, but the fields of the config aren't very well documented it seems.

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[carlocab]

Could you motivate this choice of path? Is lib/docker/cli-plugins a "well-known" path (in some sense)?

[narqo]

Yes, following the original design docker/cli#1534 docker-cli scans these paths, when it searches for plugins:

Unix-like OSes:

• $HOME/.docker/cli-plugins
• /usr/local/lib/docker/cli-plugins & /usr/local/libexec/docker/cli-plugins
• /usr/lib/docker/cli-plugins & /usr/libexec/docker/cli-plugins

I think it's reasonable to keep Docker CLI's plugins, that brew manages, in the similar structure.

Note, Docker Desktop seems to amend the default list of the paths and bundles several cli-plugins into the distribution itself (e.g. buildx and compose live inside the docker desktop's bundle).

[carlocab]

We could probably replace this post_install entirely by doing something like

(lib/"docker/cli-plugins").install_symlink bin/"docker-buildx"

Any reason why we're not doing that?

[narqo]

That's indeed simpler; I updated the changes. PHAL

[chenrui333]

@narqo thanks for your first contribution to homebrew-core! 🎉

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