Add PostgreSQL container

[philtrep]

Add a PostgreSQL container, it's definitely popular enough to warrant the extra service.

We must also add an example on how to use it.

[sign0]

PostgreSQL + PostGIS

[philtrep]

@sign0 Good idea, shouldn't take much more work to add PostGIS.

@brianfajardo If you could take care of this 👍

[brianfajardo]

yeah, I'll look into adding a PostgreSQL container 😀

[philtrep]

This is closed by #94 which added the Postgres service, courtesy of @brianfajardo 👏 There's a new issue for PostGIS: #95