title: Docker compose not using external named volume on Mac
body: <p>Though I mount data on external volume, upon <code>docker-compose down</code> I am losing my data. I am using docker-compose version <code>1.14.0, build c7bdf9e</code> and docker version <code>17.06.1-ce, build 874a737</code></p>
  
  <pre><code># docker-compose.yml
  version: '3'
  services:
    db:
      image: postgres
      volumes:
        - data:/var/lib/postgres/data
    web:
      build: .
      command: bundle exec rails s -p 3000 -b '0.0.0.0'
      volumes:
       - .:/myapp
      ports:
       - "3000:3000"
      depends_on:
       - db
  
  volumes:
    data:
      external: true
  </code></pre>