Add checkout code action to make DH readme updates possible

.github/workflows/docker-publish-master.yml

@@ -13,6 +13,11 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+      with:
+        submodules: recursive
+
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v1