Howto pass POSTGRES_USER env variable when using docker-compose .yml for docker swarm

I'm starting a docker swarm with a PostgreSQL image. I want to create a user named 'numbers' on that database.

This is my docker-compose file. The .env file contains POSTGRES_USER and POSTGRES_PASSORD. If I ssh into the container hosting the postgres image, I can see the variables when executing env. But psql --user numbers tells me that role "numbers" does not exists.

How should I pass the POSTGRES_* vars so that the correct user is created?

version: '3'

services:

  postgres:
    image: 'postgres:9.5'
    env_file:
       - ./.env
    ports:
      - '5432:5432'
    volumes:
      - 'postgres:/var/lib/postgresql/data'
    deploy:
      replicas: 1
    networks:
      - default
    restart: always

This creates the postgresql user as expected.

 $ docker run --name some-postgres -e POSTGRES_PASSWORD=mysecretpassword -e POSTGRES_USER=numbers -d postgres