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

Summary:

• We unfortunately are still on MySQL 5.7. ENG-111 tracks moving to a recent version of MariaDB
• I am purposefully deferring the question of persistence/volumes here, which is tracked in ENG-1069. I need to read up on the subject first, and also I'll probably (maybe?) want to make Redis persist as well

Test Plan: Run docker-compose up --build

Reviewers: jimpo, varun

Reviewed By: varun

Subscribers: palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3885