Add Gunicorn to Docker setup

[lordsarcastic]

For the template to be truly production ready, it needs a production-ready server setup such as Gunicorn. Gunicorn needs to be added to the server startup process to make it fully production-ready.

This setup should make use of Uvicorn alongside Gunicorn. This is to ensure Gunicorn can be easily stripped off and the template can be used with plain Uvicorn.