My understanding is that most users (especially of ROS2) will want to use the Docker Official Images located at https://hub.docker.com/_/ros, so shouldn't the README (and by extension the osrf/ros2 Docker Hub page) link to that page? I almost went through the trouble of maintaining my own ROS2 dockerfile, because the one here doesn't support ARM, and I didn't realize that the Docker Official images had builds for ROS2.