fix: ensure submodules are checked out in Docker workflow

Author: Timothy Dodd

.github/workflows/docker-image.yml

@@ -11,6 +11,8 @@ jobs:
     steps:
     - name: Checkout repository
       uses: actions/checkout@v3
+      with:
+        submodules: true
 
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v1