refs https://github.com/TryGhost/DevOps/issues/118
- we should standardize how we run MySQL, so there aren't massive gaps
in developer experience
- Docker is great for this, and it's pretty easy to set up
- this adds a docker-compose.yml file with a small MySQL setup
- also configures `yarn setup` to spin up the Docker container and add
it to config.local.json
- in the near future, we'll provide a base SQL file in .github/scripts/mysql-preload
to ensure the DB is in a known state
