add path context to docker/build-push-action

The explicit checkout action (instead of the implicit git context)
allows for passing a path context to the build-push-action which allows
file mutation during the workflow.

This should now be able to use the generated `Containerfile` from the
previous `Run beaker` step.

Author: foxxx0

.github/workflows/build.yml

@@ -60,6 +60,7 @@ jobs:
       - name: Build
         uses: docker/build-push-action@v7.1.0
         with:
+          context: .
           file: Containerfile
           #provenance: true
           push: false