To run additional (debugging) nodes on your computer, or watch the content of certain topics, you have to change the ROS_MASTER_URI (temporary) to the ip-address of the simulation container (which is running the rosmaster). The ip-address of the simulation container is set to 172.20.0.5.
In a terminal, change to ROS_MASTER_URI by:
ROS_MASTER_URI=http://172.20.0.5:11311
In the same terminal, you can now check for the running topics by rostopic list, echo a specific topic by rostopic echo <topic_name> or start nodes like rosrun rviz rviz. When you close the terminal, the ROS_MASTER_URI is set to the default value.