title: Pushing docker image to docker hub private repository return authentication error
body: <p>I have built a test image and tried to push it to docker hub's private repository. I was able to login with "docker login" and the credential gets stored in .dockercfg file. However, when I am trying to push the image with "docker push accountname/repositoryname" command it then returns the following error.</p>
  
  <blockquote>
    <p>36423578012b: Image push failed
    FATA[0006] Error pushing to registry: Authentication is required.</p>
  </blockquote>
  
  <p>What is the proper way to push to docker hub's private repository?</p>
  
  <p>Docker version:</p>
  
  <blockquote>
    <p>Client version: 1.6.0<br>
    Client API version: 1.18<br>
    Go version (client): go1.4.2<br>
    Git commit (client): 4749651<br>
    OS/Arch (client): darwin/amd64<br>
    Server version: 1.6.0</p>
  </blockquote>