title: Docker pull/push using private registry without prefixing
body: <p>We are using Nexus3 docker groups (combining dockerhub proxy and private registry) aka:</p>
  
  <pre><code>docker pull dockerproxy:5002/busybox
  </code></pre>
  
  <p>trying to pull from dockerhub if it does not exist in private registry, push storing in private registry part. Works fine.</p>
  
  <p>But how can I make:</p>
  
  <pre><code>docker pull busybox
  </code></pre>
  
  <p>go to this registry without having to prefix dockerproxy:5002 first?</p>
  
  <p>An environment variable?, $HOME/.docker/config.json? Creative tagging?</p>
  
  <p>Thanks</p>