ci(docker): Run the Docker build as part of PR checks

To prevent that pull requests can break the Docker builds, change the
Docker build workflow to also be triggered by PRs and the merge queue.
In this case, do not push the images to the remote registry.

Fixes #1608.

Signed-off-by: Martin Nonnenmacher <martin.nonnenmacher@bosch.com>

.github/workflows/docker-build.yml

@@ -1,6 +1,11 @@
 name: Docker Build
 
 on:
+  merge_group:
+    types: [ checks_requested ]
+  pull_request:
+    branches:
+      - main
   push:
     branches:
       - main
@@ -131,7 +136,8 @@ jobs:
       with:
         context: ${{ matrix.docker.context }}
         file: ${{ matrix.docker.context }}/${{ matrix.docker.dockerfile }}
-        push: true
+        push: ${{ github.event_name != 'pull_request' && github.event_name != 'merge_group' }}
+        load: ${{ github.event_name == 'pull_request' || github.event_name == 'merge_group' }}
         tags: ${{ steps.meta-base.outputs.tags }}
         labels: ${{ steps.meta-base.outputs.labels }}
         cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ github.repository_owner }}/ort-server-${{ matrix.docker.image }}:cache
@@ -160,4 +166,6 @@ jobs:
           -Djib.console=plain \
           -Djib.container.labels="$(echo "${{ steps.meta.outputs.labels }}" | tr '\n' ',' | sed 's/,$//')" \
           -Djib.to.tags="$(echo "${{ steps.meta.outputs.tags }}" | tr '\n' ',' | sed 's/,$//')"
-        docker push ${{ env.REGISTRY }}/${{ github.repository_owner }}/ort-server-${{ matrix.docker.jibImage }} --all-tags
+        if [ "${{ github.event_name }}" != "pull_request" ] && [ "${{ github.event_name }}" != "merge_group" ]; then
+          docker push ${{ env.REGISTRY }}/${{ github.repository_owner }}/ort-server-${{ matrix.docker.jibImage }} --all-tags
+        fi