Feature request: git submodule context

[pl4nty]

I build several git submodules in a monorepo. It would be helpful to use the submodule git context for generating metadata, instead of the monorepo's context. A context-path option might work, or assuming context is a git path if it isn't git or workflow.

[crazy-max]

I think we could add a separator to the git context following by the path you want to use. Something like:

      -
        name: Docker meta
        id: meta
        uses: docker/metadata-action@v4
        with:
          context: "git:foo"

Where foo is a submodule.

But then GitHub metadata attached to the Git context would be quite wrong imo so I'm not sure.