chore: align to existing gha pattern in repo

.github/workflows/build-image.yml → .github/workflows/publish-image-test.yml

@@ -1,6 +1,11 @@
-name: Build Docker image
+name: Test building Docker image
 
-on: pull_request
+on:
+  pull_request:
+    branches:
+      - master
+    # Review gh actions docs if you want to further define triggers, paths, etc
+    # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#on
 
 jobs:
   build_image: