title: How can I extend an image, so that it still works? When can an image be in the from line in a Dockerfile?
body: <p>I have a sshd Dockerfile. It works. I can ssh to it. I called the image <code>local/c7-sshd</code>.</p>
  
  <pre><code>from centos:7
  ...
  expose 22
  cmd ["/usr/sbin/sshd","-D"]
  </code></pre>
  
  <p>Now I want to extend this image to a httpd image. I have a Dockerfile,</p>
  
  <pre><code>from local/c7-sshd
  ...
  expose 80
  cmd ["/usr/sbin/httpd","-D","FOREGROUND"]
  </code></pre>
  
  <p>When I start a container from this image, httpd runs, but sshd not. </p>
  
  <p>So I do not know when or not an image can be after <code>from</code> in a Dockerfile.</p>
  
  <p>Thanks.</p>