docker: Further refine logic to apply migrations

docker: Further refine logic to apply migrations

We don't want to apply migrations by default since this will cause WIP
migrations to be applied if someone runs e.g. 'docker-compose run'. What
we really want to check is "are there any migrations currently applied?"
and skip if so, but Django doesn't provide an easy way to do this, so we
instead check for a random Django application.

Signed-off-by: Stephen Finucane <stephen@that.guru>