Allow dockers to use remote ROS_MASTER_URI

[Raymond]

Suggestion to enable this functionality is to change

docker run --network host --gpus all --name $1_backend --rm -it $image_name 

in line to

ROS_URI="${ROS_MASTER_URI:-"localhost:11311"}
docker run --network host --gpus all --name $1_backend --rm -it $image_name -e ROS_MASTER_URI="${ROS_URI}"

[Raymond]

@Melanoneiro please merge if testing is successful.