[future] Upgrade the backend Docker container to Python 3.6+

[MikeTheCanuck]

Based on our limited testing of Linux developer machines in Issue #81, it appears that at present, Linux hosts cannot successfully build the docker container, so we'll go with Python 3.5 for now.

If it becomes necessary or desirable to use Python 3.6 or later, we'll have to re-investigate the ability of Linux hosts to build the docker container locally.