.github/workflows/build.yaml: add pull request CI

Author: classabbyamp

.github/workflows/build.yaml

@@ -3,7 +3,12 @@ name: 'Build containers'
 
 on:
   workflow_dispatch:
+  pull_request:
+    branches:
+      - master
   push:
+    branches:
+      - master
     paths-ignore:
       - COPYING
       - README.md
@@ -53,6 +58,7 @@ jobs:
         uses: docker/setup-buildx-action@v2
 
       - name: Login to GCHR
+        if: github.event_name != 'pull_request'
         uses: docker/login-action@v2
         with:
           registry: ghcr.io
@@ -63,7 +69,7 @@ jobs:
         id: build_and_push
         uses: docker/bake-action@v3
         with:
-          push: ${{ endsWith(github.ref, 'master') }}
+          push: ${{ github.event_name != 'pull_request' }}
           files: |
             docker-bake.hcl
             ${{ steps.meta.outputs.bake-file }}