Use the official postgres docker image

The official postgres container does not provide environment variables to create the default user/db.
It uses the **postgres** user and it is set in *pg_hba.conf* to never ask for a password (trust everything).
It also creates an empty database called *'postgres'* that we remove in this script and create a new one called *'cloudbreak'* instead.
Using this container in a production environment is not suggested with its default settings but for development purposes it is OK.