The reason I didn't use the sqlite3 default here is to make the container stateless. That makes it much easier to deploy and upgrade: Just rebuild the image and point it to your existing database.
But I agree that this is a more advanced setup. So I suggest I'll change it so it will use sqlite3 by default and use a data volume to separate state from application (Docker stuff: http://docs.docker.io/en/latest/use/working_with_volumes/)

Regarding installing alternative drivers: Good point, I don't see a way to support using alternative drivers beside those already installed. I think for the docker case it would make sense to just install all (common) database drivers. Can this be done without changing the packages.json?