title: Unable to add docker GPG key via apt-key
body: <p>i enter a command for add docker's official GPG key but i don't have OK in output.</p>
  
  <p>The command is :</p>
  
  <pre><code>$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
  </code></pre>