title: how to update docker in coreos
body: <p>I typed <code>docker version</code> into a newly provisioned coreos instant on ec2 and I got this message</p>
  
  <pre><code>Client version: 0.6.3
  Go version (client): go1.1.2
  Git commit (client): b0a49a3
  Server version: 0.6.3
  Git commit (server): b0a49a3
  Go version (server): go1.1.2
  Last stable version: 0.7.3, please update docker
  </code></pre>
  
  <p>how do I update docker?</p>