title: specify max log json file size in docker compose
body: <p>I am trying to specify the max file size for <code>json-file</code> in <code>docker-compose.yml</code>, like this,</p>
  <pre><code>log-opt:
    max-size=50m
  </code></pre>
  <p>but when I tried to <code>docker-compose up</code>, it threw me an error,</p>
  <blockquote>
  <p>ERROR: In file './docker-compose.yml', service 'log-opt' must be a mapping not a string.</p>
  </blockquote>
  <p>How to fix it?</p>
  <p>ps. I am using <code>docker 1.11.2</code></p>