ci: adding a check-docker job in CI.

[xiao-jay]

Suggest adding a check-docker job in CI.

  check-docker:
    name: Check Docker image
    needs: [lint-build]
    runs-on: ubuntu-20.04
    steps:
      - uses: actions/checkout@v3
      - uses: docker/build-push-action@v3
        with:
          context: .
          push: false

from #21 (comment)