title: Execute command on host during docker build
body: <p>Is it possible to create <code>Dockerfile</code> that executes a command on host when image is being build?</p>
  
  <p><strong>Now I'm doing:</strong></p>
  
  <pre><code>./script_that_creates_magic_file.sh
  docker build .
  </code></pre>
  
  <p>with Dockerfile:</p>
  
  <pre><code>FROM alpine
  COPY magic_file
  </code></pre>
  
  <p><strong>I want to be able to do:</strong></p>
  
  <pre><code>docker build .
  </code></pre>
  
  <p>with Dockerfile:</p>
  
  <pre><code>FROM alpine
  # invoke script_that_creates_magic_file.sh on the host
  COPY magic_file
  </code></pre>
  
  <p>Of course, this script is in the same directory as Dockerfile.</p>