Add "rm -rf /var/lib/apt/lists/*" to ubuntu Dockerfile

21M worth of files could be removed from the ubuntu image if rm -rf /var/lib/apt/lists/* was added to the Dockerfile.

More info can be found on this page: https://docs.docker.com/engine/articles/dockerfile_best-practices/

In addition, cleaning up the apt cache and removing /var/lib/apt/lists helps keep the image size down. Since the RUN statement starts with apt-get update, the package cache will always be refreshed prior to apt-get install.

Edit: It should be added at the end of both of the RUN commands.