Suggestion to use POSTGRES_USER=postgres

[mitirmizi]

Context:
https://github.com/MobilityDB/MobilityDB-docker/blame/71b69de9a041fdd8fd8705068bab98403644c95c/14-3.2-develop/Dockerfile#L7

We are creating a new user called docker, would not the default user postgres be a better choice instead?
People will be already used to using this user, so it will make their experience user-friendly.