version: '2'
services:
  db.postgres:
    container_name: db.postgres
    image: postgres:10
    environment:
      - POSTGRES_USER:'myuser'
      - POSTGRES_PASSWORD:'myuserpassword'
      - POSTGRES_DB:'mydb'
    ports:
      - '5432:5432'
    volumes:
      - ./pgdata:/var/lib/postgresql/data

docker exec -it db.postgres psql -U myuser myuserpassword
psql: FATAL:  role "myuser" does not exist

$ docker exec -it db.postgres /bin/bash
root@1a0531e0350f:/# psql -U myuser
psql: FATAL:  role "myuser" does not exist