title: Got an error when running rake db:create in Docker
body: <p>I'm new in Docker then followed their official tutorial in running Ruby on Rails and Postgresql. Everything is okay when I run <code>docker-compose up</code> but when I tried the command <code>docker-compose run web rake db:create</code>, I got an error of  <code>FATAL:  role "root" does not exist</code>.</p>
  
  <pre><code>database.yml
  
  default: &amp;default
    adapter: postgresql
    encoding: unicode
    pool: 5
    host: db
  
  development:
    &lt;&lt;: *default
    database: myapp_development
  
  test:
    &lt;&lt;: *default
    database: myapp_test
  
  production:
    &lt;&lt;: *default
    database: myapp_production
    username: postgres
    password:
  </code></pre>