gh-actions: Run publish action on release branches (#41)

Previously the action for building the image with the tag defined in
`releasing/VERSION` would only run from commits merged in master.

This action should also get triggered from release branches as well.

Signed-off-by: Kimonas Sotirchos <kimwnasptd@gmail.com>

.github/workflows/docker-publish-releasing.yml

@@ -5,6 +5,7 @@ on:
     # Publish `master` as Docker `latest` image.
     branches:
       - master
+      - release-*
 
     paths:
       - releasing/VERSION