Add Docker support to make development/deployment simpler

[programdenizen]

With a Dockerfile for the DB, and one for the bot app, and a couple of docker-compose files (one for local development, and one for app deployment), onboarding potential contributors and deploying the bot can be made much simpler.

More documentation on what is needed to set up this bot on a Discord server to test changes wouldn't hurt either, unless we want some barriers to entry, to prevent having to hold potential contributor's hands.