title: Docker build ADD vs RUN curl
body: <p>If I run a dockerfile where I have a command like</p>
  
  <pre><code>RUN curl -o file.txt http://X.X.X.X/path/to/file/file.txt
  </code></pre>
  
  <p>the build works, whereas if I use</p>
  
  <pre><code>ADD http://X.X.X.X/path/to/file/file.txt file.txt
  </code></pre>
  
  <p>The build fails and it complains about</p>
  
  <pre><code>Got HTTP status code &gt;= 400: 503 Service Unavailable
  </code></pre>
  
  <p>Is there something about ADD that I'm not understanding?</p>
  
  <p><strong>Edit</strong> the file is also accessible through the docker host.</p>