[keyserver] Add MySQL config to docker-compose.yml

Summary:
Now that we can specify MySQL config via environmental variable, we are ready to set it up in Docker Compose.

Depends on D4102

Test Plan:
I created a keyserver/.env file with the following contents:

COMM_MYSQL_HOST=database
COMM_MYSQL_DATABASE=comm2
COMM_MYSQL_USER=comm2
COMM_MYSQL_PASSWORD=pass2

Then I ran docker-compose down -v && docker-compose up --build. The keyserver still crashed because the database tables weren't created yet, but it was able to connect to MySQL.

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4103