title: Login to Docker Hub by command line
body: <p>I have read some Docker tutorials and I see this command line:</p>
  <pre><code>docker login -u LOGIN -p PASSWORD
  </code></pre>
  <p>But the registry server URL is never set.
  How does the <code>docker</code> command know the registry URL? What is the URL for Docker Hub Registry? I have tried this:</p>
  <pre><code>docker login -u LOGIN -p PASSWORD cloud.docker.com
  </code></pre>
  <p>but it does not work.</p>