Centralized configuration and supervisor

This commit does two things:

1. Configurations that were spread across docker/*.conf are now in a single
   config file and passed to the container using environment variables.

2. The three processes in the container (MySQL, Apache, and Racket) are now
   managed by supervisord. This will keep the system running when things crash.

The SSL configuration files still need to be manually hacked in.
Merge pull request #2 from brownplt/config

Centralized configuration and supervisor