title: run docker build in cmake
body: <p>I'm trying to build the docker image in cmake with the following:</p>
  
  <pre><code>add_custom_target(module_tests
    COMMAND cp -r ${FFMPEG} .
    COMMAND cp ${FFMPEG_SO_INSTALL} ${FFMPEG}/lib
    COMMAND docker build -t moduletest -f Dockerfile .
  )
  </code></pre>
  
  <p>But I got error saying <code>/bin/sh: 1: docker: not found</code>, while running docker build manually works.</p>