Generic docker image for dev/prod

[JAForbes]

I've taken to having a migration container as it just fits into a docker-compose workflow well. It'd be nice to not need to do as much config in each project and just use a premade image and point the volume with the migration scripts in the right place.

Should give the user the option to have their own package.json and dependencies as it's useful to pull in things like dotenv in a migration and the docker image shouldn't be too prescriptive.

It should also make it easier for non-node users to take advantage of pgmg without having to install npx / npm on their host machine.