CI: Update image release workflow to limit release scope

This reverts the PR trigger which was used for testing, and now
ensures the image will only be released on a PR merged to main
since we're not allowing for a direct push to main.

.github/workflows/build_and_publish_docker_image.yml

@@ -4,9 +4,7 @@ on:
   push:
     branches:
       - main
-  pull_request:
-    branches:
-      - main
+
 
 jobs:
   build-and-push: