🚧 build: Check out explicitly to avoid build-push-actions trying to get the submodules

.github/workflows/build.yml

@@ -11,6 +11,12 @@ jobs:
   docker:
     runs-on: ubuntu-latest
     steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+        # We are checking out explicitly, so build-push-action isn't trying
+        # to checkout the (unreachable) submodule. (Using "context" there.)
+
       -
         name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v2
@@ -18,5 +24,6 @@ jobs:
         name: Build
         uses: docker/build-push-action@v4
         with:
+          context: .
           file: Dockerfile-core
           push: false