- use same postgres container for dev and test
  (create 2 different databases)
- use docker-compose.override.yml to avoid duplication
- ignore more files not needed in the docker image

By default docker-compose reads both docker-compose.yml and
docker-compose.override.yml.
In docker-compose.override.yml we mount the current directory as
a volume for development.

To test the image we only use docker-compose.yml:
- don't force port 5432 to the outside world to avoid conflict
- do not mount the current directory