title: Postgres Docker image is not creating database with custom name
body: <p>The documentation of the <code>postgres</code> Docker image <a href="https://hub.docker.com/_/postgres/#postgres_db" rel="noreferrer">says</a> the following about the env var <code>POSTGRES_DB</code>:</p>
  
  <blockquote>
    <p>This optional environment variable can be used to define a different name for the default database that is created when the image is first started. If it is not specified, then the value of POSTGRES_USER will be used.</p>
  </blockquote>
  
  <p>I have found that this is not true at all. For example, with this config:</p>
  
  <pre><code>version: '3.7'
  
  services:
    db:
      image: postgres:11.3-alpine
      restart: always
      container_name: store
      volumes:
        - postgres_data:/var/lib/postgresql/data/
      ports:
        - 5432:5432
      environment:
        - POSTGRES_USER=custom
        - POSTGRES_DB=customname
        - POSTGRES_PASSWORD_FILE=/run/secrets/db_password
      secrets:
        - db_password
  
  volumes:
    postgres_data:
  
  secrets:
    db_password:
      file: config/.secrets.db_password
  </code></pre>
  
  <p>The default database is called <code>postgres</code>, and not <code>customname</code> as I have specified:</p>
  
  <pre><code>$ docker exec -it store psql -U custom customname
  psql: FATAL:  database customname does not exist
  $ docker exec -it store psql -U custom postgres
  psql (11.3)
  Type help for help.
  
  postgres=# ^D
  </code></pre>
  
  <p>Am I missing something obvious?</p>