Explore alternative base Docker images

[bzier]

Currently, the project is based on ubuntu:xenial-20170915, but this results in pretty large images, especially after installing all dependencies. We may be able to switch to some alternative image with a smaller footprint.

Currently the build is split into multiple phases - one for building the input plugin from source, and one for supporting the actual gym environment. These do not necessarily need to be based on the same underlying image, so could differ if needed.

Need to ensure that we have support for at least:

• building the C mupen64plus-input-bot plugin
• installing and running mupen64plus
• Python3 (see Upgrade to Python3 (Docker image) #81)
• XVFB, VirtualGL, x11vnc