Postgres image now requires `POSTGRES_PASSWORD`

[ewjoachim]

Source: docker-library/postgres#658 (review)

This is a breaking change, but I think it's one that makes sense overall (and that the benefit outweighs the pain in this instance).

We need to add a password in docker-compose.yml and make sure it is passed properly in the contribution doc (CONTRIBUTING.rst)

[ewjoachim]

@charlesaracil-ulti up for it ?

[charlesaracil-ulti]

@ewjoachim of course !

[ewjoachim]

Closed by #177