[META] Docker images refactor

[Agustin Alba Chicar]

To make a better use of Docker images that OSRF already provides, we should do the following:

• Replace gazebo8 dockerfiles by using ORSF images.
• Download, via Dockerfile, Gazebo resources to make them availble once the container is run.
• Create a uniform Dockerfile installing common dev tools (similar to ekudev).
• Improve integration with nvidia-docker.
• Evaluate running Gazebo headless tests to improve testing.
• Upload images to Docker Hub (#160).