Integration of dockerized ROS nodes into gitlab-ci

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What ROS specifics (environment variables, etc.) need to be considered if containerized ROS nodes shall be run in gitlab-ci (docker container runner)? It's probably about the network setup only which needs to be set explicitly in the CI context. According to Exposing ROS Containers to Host Machine the ROS_IP, ROS_HOSTNAME and ROS_MASTER_URI need to be set. Anyway either I am missing something or I have an error in my setup.

Side note: I am NOT INTERESTED in running jobs for single ROS packages in gitlab-ci like described here (with corresponding example). Instead of running jobs in a ROS docker image like e.g. image: ros:melodic-robot I am INTERESTED in to run the job in an image: docker and run several docker containers in there...