title: ERROR: In file './docker-compose.yml', volume must be a mapping not a string
body: <p><strong>Question:</strong>
  Why do I get this error?</p>
  <blockquote>
  <p>ERROR: In file './docker-compose.yml', volume 'mariavolume' must be a mapping not a string.</p>
  </blockquote>
  <p>My docker-compose file is almost identical to this one: <a href="https://docs.docker.com/compose/wordpress/" rel="nofollow noreferrer">https://docs.docker.com/compose/wordpress/</a></p>
  <pre class="lang-yaml prettyprint-override"><code>version: '2'
  services:
    wordpress:
      image: wordpress:latest
      restart: always
      depends_on:
        - db
      ports:
        - 8080:80
      environment:
        WORDPRESS_DB_PASSWORD: example
        WORDPRESS_DB_HOST: 3306
    db:
      image: mariadb
      restart: always
      environment:
        MYSQL_ROOT_PASSWORD: example
      volumes:
        - maria_volume: /var/lib/mysql
  volumes:
    maria_volume: ~/mariadb
  </code></pre>