feat(integration): run integration test in pr builds

multi-arch with --load doesn't work, so add a separate step using the local platform to
make an image available for testing.

see docker/buildx#59

.github/workflows/pr.yml

@@ -73,3 +73,16 @@ jobs:
           tags: |
             "${{ env.CONTAINER_REGISTRY }}/${{ steps.build_variables.outputs.REPO }}:latest-java11-ubuntu"
             "${{ env.CONTAINER_REGISTRY }}/${{ steps.build_variables.outputs.REPO }}:${{ steps.build_variables.outputs.VERSION }}-java11-ubuntu"
+      - name: Build local slim container image for testing
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: Dockerfile.slim
+          load: true
+          platforms: local
+          tags: |
+            "${{ steps.build_variables.outputs.REPO }}:${{ steps.build_variables.outputs.VERSION }}"
+      - name: Test local slim container image
+        env:
+          FULL_DOCKER_IMAGE_NAME: "${{ steps.build_variables.outputs.REPO }}:${{ steps.build_variables.outputs.VERSION }}"
+        run: ./gradlew ${{ steps.build_variables.outputs.REPO }}-integration:test