Add Docker build CI to pull requests

.github/workflows/ci.yml

@@ -4,6 +4,9 @@ on:
   push:
     branches:
       - main
+  pull_request:
+    branches:
+      - main
 
 jobs:
   docker:
@@ -26,7 +29,7 @@ jobs:
         id: docker_build
         uses: docker/build-push-action@v2
         with:
-          push: true
+          push: ${{ github.event_name == 'push' }}
           build-args: GITHUB_SHA
           tags: ekzhang/rustpad:latest