Restore checkout action, needed for yaml access

.github/workflows/build_docker_hub.yml

@@ -8,6 +8,9 @@ jobs:
   build_for_docker_hub:
     runs-on: ubuntu-latest
     steps:
+      - name: Check out code
+        uses: actions/checkout@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3