Feature request: Allow specifying subdirectory with default git context

When using git repository contexts, Docker allows specifying a subdirectory using the syntax <repo>#<ref>:<subdir>, as documented here:

• https://docs.docker.com/engine/reference/commandline/build/#git-repositories

BuildKit used to not support this feature, but as of BuildKit 0.9.0, it does:

• Release notes: https://github.com/moby/buildkit/releases/tag/v0.9.0
• https://github.com/moby/buildkit/issues/1684
• https://github.com/moby/buildkit/pull/2116

Can this action add support for this, to avoid the need to use checkouts to build subdirectories?

Workaround

For the time being, specifying a full git context seems to work; for example, to build subdirectory spam:

      - uses: docker/build-push-action@v2
        with:
          context: "https://github.com/${{ github.repository }}.git#${{ github.sha }}:spam"

However, this doesn’t use the same logic for determining the default git context as the action does, so it might be fragile.

[simensol]

@PiDelport Do you have an example on how to add credentials to the URL?

[PiDelport]

Yes, for private repositories, you’ll probably need to add credentials from an appropriate environment variable to the URL.