title: Usage of env variable in docker compose run command
body: <p>Running the command <code>docker-compose run -e TYPE=result mongo_db_backup</code> should give me the value of the given TYPE variable:</p>
  
  <pre><code>mongo_db_backup:
    image: 'mongo:3.4'
    volumes:
      - '/backup:/backup'
    command: sh -c '$$(echo $TYPE)'
  </code></pre>
  
  <p>But instead I get the error <code>The TYPE variable is not set. Defaulting to a blank string.</code> What am I doing wrong</p>