Dockerize for production & other server changes

[DakuTree]

At the moment we are only using Docker for Development, but it may be worth looking at using it for production as well. This should simplify deployment as well as keep things like server settings more easily commitable and so on.

While doing this, I'd also like to migrate to Nginx and MariaDB (from Apache and MySQL). I've been using Nginx for other projects and it's just much nicer to use, and MariaDB seems to be the way forward over MySQL. I'd also like to bump up to PHP7.3 as it was released a month ago.

Checklist:

• Make a working production Dockerfile/docker-compose.
• Migrate to Nginx.
• Migrate to MariaDB.
• Migrate to PHP7.3.

Other notes:
Watchtower should also be used to keep Docker containers up to date. Certbot container would be handy too.