Postgres Docker image is not creating database with custom name : <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>
