Test pull requests in CI

.github/workflows/publish-image.yaml

@@ -4,6 +4,8 @@ on:
   push:
     branches: ['master']
     tags: ['*']
+  pull_request:
+    branches: ['master']
 
 env:
   DOCKER_HUB_USER: phlak
@@ -34,6 +36,6 @@ jobs:
       - name: Build & Push Image
         uses: docker/build-push-action@v2
         with:
-          push: true
+          push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.extract-metadata.outputs.tags }}
           labels: ${{ steps.extract-metadata.outputs.labels }}