Dockerfile for backend

[ethanwillis]

Create a dockerfile that can run the Backend in a container. Refactor all necessary environment variables into either this dockerfile or the eventual docker-compose file that ties all of the docker files for backend, gui, daemon, and database together. See: #1

[ethanwillis]

Work for this is happening here: https://github.com/ethanwillis/SpeedSeed3/tree/docker