Enable image building in pull requests

.github/workflows/build-images.yml

@@ -1,9 +1,7 @@
 name: Build and release images
 
-on:
-  push:
-    branches:
-      - main
+on: push
+
 concurrency: build-${{ github.sha }}
 
 jobs:
@@ -28,13 +26,15 @@ jobs:
           docker tag ghcr.io/quiltmc/mongodb-s3-backup:latest ghcr.io/quiltmc/mongodb-s3-backup:${{ steps.sha_tag.outputs.tag }}
 
       - name: Login to Github Container Registry
+        if: github.ref == 'refs/heads/main'
         uses: docker/login-action@v1
         with:
           registry: ghcr.io
           username: ${{ github.repository_owner }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Push images to the registry
+        if: github.ref == 'refs/heads/main'
         run: |
           docker push ghcr.io/quiltmc/postgresql-s3-backup:latest
           docker push ghcr.io/quiltmc/postgresql-s3-backup:${{ steps.sha_tag.outputs.tag }}