before docker run command added git submoulde update command in docker build Updated README.md (#10878)

there is dependency on submodule libs/hbb_common, 
we need to initialize submodule after cloning repo and before running container

README.md

@@ -127,6 +127,7 @@ Begin by cloning the repository and building the Docker container:
 ```sh
 git clone https://github.com/rustdesk/rustdesk
 cd rustdesk
+git submodule update --init --recursive
 docker build -t "rustdesk-builder" .
 ```