Publish a package tagged latest.

Each push/merge to master will now trigger package build and publish
action. The published package will be tagged `latest`.

Author: dandanlen

.github/workflows/publish.yml

@@ -2,6 +2,9 @@ name: Publish Docker image
 on:
   release:
     types: [published]
+  push:
+    branches:
+      - master
 jobs:
   push_to_registry:
     name: Push Docker image to GitHub Packages