Create GitHub action.yml

As per docs:
- https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#runs-for-docker-container-actions

Signed-off-by: Victor Sollerhed <[email protected]>

Author: MPV

action.yml

@@ -0,0 +1,6 @@
+name: container-diff
+
+runs:
+  using: 'docker'
+  image: 'action/Dockerfile'
+