Add git submodule update command to docker build process documentation

docker/README.md

@@ -5,6 +5,7 @@
 Before building the image, you need to have built the unmanic python package:
 ```bash
 rm -rfv ./build && rm -fv ./dist/unmanic-*
+git submodule update --init --recursive
 python3 ./setup.py sdist bdist_wheel
 ```