Self-hosting via docker added

Multiple people have been asking about it, so the support for self-hosting using docker is added.

The quickest way to get it up and running is using docker-compose: https://github.com/RikudouSage/LemmySchedule/#docker-compose

In the coming days I'll be adding support for locking the app to a single instance only, meaning if you self host it alongside your Lemmy instance, only people from your own instance will be able to use it. Done!