title: Getting "FATAL: role "root" does not exist" when trying to do "docker up"
body: <p>I'm starting to work with an existing Rails project that uses Docker. I've used Rails for a long time but never Docker.</p>
  
  <p>After I do a <code>docker build .</code> I try to do a <code>docker-compose up</code>, but I get:</p>
  
  <blockquote>
    <p>FATAL:  role "root" does not exist
    /usr/local/bundle/gems/activerecord-4.2.5.2/lib/active_record/connection_adapters/postgresql_adapter.rb:661:in `rescue in connect': FATAL:  role "root" does not exist (ActiveRecord::NoDatabaseError)</p>
  </blockquote>
  
  <p>It seems to me that the Docker machine is probably trying to connect to the database as the <code>root</code> user, but there's no role called <code>root</code>, so the connection is rightly failing.</p>
  
  <p>The thing I don't know is why Docker is apparently trying to connect to the database as <code>root</code> and how to get it to use the right user.</p>
  
  <p>Here's my <code>database.yml</code>:</p>
  
  <pre><code>development:
    database: my_app_development
    adapter: postgresql
    encoding: unicode
    pool: 5
  </code></pre>
  
  <p>Any help is appreciated.</p>
  
  <p>Edit: here's my <code>docker-compose.yml</code>:</p>
  
  <pre><code>web:
    build: .
    volumes:
      - .:/my_app
    ports:
      - "3000:3000"
    links:
      - postgres
      - redis
      - mailcatcher
    env_file:
      - 'config/application.yml'
  postgres:
    image: postgres:9.4
    ports:
      - "5432"
    env_file:
      - 'config/database.yml'
  redis:
    image: redis:3.0.6
  mailcatcher:
    image: schickling/mailcatcher
    ports:
      - "1080:1080"
  </code></pre>