title: Unable to use docker due to ZScaler and certificate issues
body: <p>I have VMware Photon OS running in VMware Player. This will be used as the host OS to run Docker containers.</p>
  
  <p>However, since I'm behind a ZScaler, I'm having issues running commands that access external resources. E.g. <code>docker pull python</code> gives me the following output (I added some line breaks to make it more readable):</p>
  
  <pre><code>error pulling image configuration: 
  Get https://dseasb33srnrn.cloudfront.net/registry-v2/docker/registry/v2/blobs/sha256/a0/a0d32d529a0a6728f808050fd2baf9c12e24c852e5b0967ad245c006c3eea2ed/data
  ?Expires=1493287220
  &amp;Signature=gQ60zfNavWYavBzKK12qbqwfOH2ReXMVbWlS39oKNg0xQi-DZM68zPi22xfDl-8W56tQmz5WL5j8L39tjWkLJRNmKHwvwjsxaSNOkPMYQmhppIRD0OuVwfwHr-
  1jvnk6mDZM7fCrChLCrF8Ds-2j-dq1XqhiNe5Sn8DYjFTpVWM_
  &amp;Key-Pair-Id=APKAJECH5M7VWIS5YZ6Q: 
  x509: certificate signed by unknown authority
  </code></pre>
  
  <p>I have tries to extract the CA root certificates (in <code>PEM</code> format) for ZScaler from my Windows workstation, and have appended them to <code>/etc/pki/tls/certs/ca-bundle.crt</code>. But even after restarting Docker, this didn't solve the issue.</p>
  
  <p>I've read through numerous posts, most referencing the command <code>update-ca-trust</code> which does not exist on my system (even though the <code>ca-certificates</code> package is installed).</p>
  
  <p>I have no idea how to go forward. AFAIK, there are two options. Either:</p>
  
  <ul>
  <li>Add the ZScaler certificates so SSL connections are trusted.</li>
  <li>Allow insecure connections to the Docker hub (but even then it will probably still complain because the certificate isn't trusted).</li>
  </ul>
  
  <p>The latter works by the way, e.g. executing <code>curl</code> with the <code>-k</code> option allows me to access any https resource.</p>