Run one off commands as part of local environment

[shrink]

e.g: when starting up the application, migrate the database.

There's a similar need in production (e.g: run migrate command on deploy) so perhaps there's some commonality that can be identified.

The separate container pattern is neat -- i.e: what's recommended for aws-cli sometimes -- but not sure how best to achieve that with php though, given we'd need the container to have access to all the environment variables (does docker-compose support sharing them? This might be an example where .env makes sense).