PyTerraBackTYL service in Docker image should be invoked by an WSGI (e.g. gnunicorn)

[dev-dull]

The docker image for PyTerraBackTYL currently invokes the service using Python/Flask directly. Since it is intended to be ran as an internally facing service, it is "probably fine" however, using gnunicorn is dead simple and brings the application further into alignment with best practices.