Allow building buildx from source

[crazy-max]

Closes #98

I made some changes from what we said in #98. Now buildx will be built on-the-fly against the reference passed to build-ref input. Here is a workflow:

build-ref:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        ref:
          - master
          - refs/tags/v0.5.1
          - refs/pull/648/head
    steps:
      -
        name: Checkout
        uses: actions/checkout@v2
      -
        name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v1
        with:
          build-ref: https://github.com/docker/buildx.git#${{ matrix.ref }}
      -
        name: Check version
        run: |
          docker buildx version

Run: https://github.com/docker/setup-buildx-action/runs/2963153312?check_suite_focus=true#step:3:94

cc @tonistiigi

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[crazy-max]

Binary will be cached. Atm remote ref sha is used as cache key. ${repo}-${sha} would be safer imo but it's fine in the meantime.

[tonistiigi]

Overall LGTM but the build-ref name is not great. Maybe buildx-source? Theoretically, we could also just use the version field and detect if it is a URL to a git repo.

[crazy-max]

@tonistiigi

we could also just use the version field and detect if it is a URL to a git repo.

Ok I've changed the version input behavior to handle a git context if we want to build from source.