Build (only) container images for new PRs (#3788)

.github/workflows/ci-container-image.yml

@@ -1,9 +1,10 @@
-name: Build and push latest image to container registry
+name: container-image
 
 on:
   push:
-    branches:
-      - main
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
   schedule:
     # Ideally, we would update the image every time the base image
     # (p4lang/behavioral-model) is updated, this is good enough.
@@ -31,14 +32,14 @@ jobs:
         if [[ "$GITHUB_REF" =~ "main" ]]; then
             TAG="latest"
         else
-            echo "Invalid Github ref $GITHUB_REF"
-            exit 1
+            TAG=${GITHUB_REF////_}
         fi
         echo "Tag is $TAG"
         echo "::set-output name=tag::$TAG"
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v1
     - name: Login to DockerHub
+      if: ${{ github.event_name != 'pull_request' }}
       uses: docker/login-action@v1
       with:
         username: ${{ secrets.DOCKER_USERNAME }}
@@ -47,7 +48,7 @@ jobs:
       uses: docker/build-push-action@v2
       with:
         context: .
-        push: true
+        push: ${{ github.event_name != 'pull_request' }}
         tags: p4lang/p4c:${{ steps.get-tag.outputs.tag }}
         cache-from: type=gha
         cache-to: type=gha,mode=max