Update Dockerfile to use correct repo

ROS 2 tags have been migrated to the single ROS repo
See tags at https://hub.docker.com/r/osrf/ros/tags

Context:

• https://discourse.ros.org/t/announcing-official-docker-images-for-ros2/7381
• https://github.com/docker-library/docs/pull/1381#pullrequestreview-185320039