title: could not translate host name "postgres" to address
body: <p>I use PostreSQL and Django 1.8 in my app.
  I have an error:</p>
  
  <pre><code>django.db.utils.OperationalError: could not translate host name "postgres" to address: nodename nor servname provided, or not known
  </code></pre>
  
  <p>but I cannot find a please where host name "postgres" is set, because I have set host name to localhost.</p>
  
  <pre><code>DATABASE_USER = MY_DATABASE_USER = admin
  DATABASE_HOST = MY_DATABASE_HOST = localhost
  
  DATABASES = {
      'default': {
          'ENGINE': 'django.db.backends.postgresql_psycopg2',
          'NAME': 'default_db',
          'USER': os.getenv('DATABASE_USER', ''),
          'PASSWORD': os.getenv('DATABASE_PASSWORD', ''),
          'HOST': os.getenv('DATABASE_HOST', ''),
          'PORT': '5432',
      },
      'my_db': {
          'ENGINE': 'django.db.backends.postgresql_psycopg2',
          'NAME': os.getenv('MY_DATABASE_NAME', 'my_dev'),
          'USER': os.getenv('MY_DATABASE_USER', os.getenv('DATABASE_USER', 'admin')),
          'PASSWORD': os.getenv('MY_DATABASE_PASSWORD', os.getenv('DATABASE_PASSWORD', '')),
          'HOST': os.getenv('MY_DATABASE_HOST', os.getenv('DATABASE_HOST', '')),
          'PORT': '5432',
      }
  }
  </code></pre>
  
  <p>Can I fix that error?</p>