Test Docker image build on pull-requests

.github/workflows/docker.yml

@@ -5,8 +5,24 @@ on:
   push:
     tags:
       - 'v*.*.*'
+  pull_request:
+    branches: [ main ]
 jobs:
+  test-image-build:
+    if: github.event.pull_request
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Build image
+        uses: docker/build-push-action@v3
+        with:
+          context: .
+          file: Dockerfile
+          push: false
+
   multiplatform-build:
+    if: ${{ ! github.event.pull_request }}
     runs-on: ubuntu-latest
     steps:
       - name: Checkout