This pull request switches testing to use an actual docker daemon, vs mocking everything out.
It may also catch actual breaking issues in our tests, which is great!
Since the database can be exposed to the outer world, it seems better to
have an encrypted connection to it.
We automatically generate a self-signed certificate since it's only used
to encrypt the connection but one can easily replace it by a custom one,
just overwrite `server.crt` and `server.key` in the
/var/lib/dokku/services/postgres/DB_NAME/data directory.
