title: php composer inside container lost vendor directory
body: <p>I have the following Dockerfile</p>
  
  <pre><code>FROM        bitgandtter/sf:php7
  
  # basic env fix
  ENV         TERM xterm
  
  # install packages
  ADD         .   /var/www
  
  # update dependencies
  RUN         cd Helpers &amp;&amp; SYMFONY_ENV=prod composer update -o --no-dev
  
  ENV         SYMFONY_ENV prod
  </code></pre>
  
  <p>After build the image the Helpers directory does not contain the vendor directory.</p>
  
  <p>I really dont know why is that since the previous compsoer update just execute successfully and the image was created just fine.</p>
  
  <p>Any help please</p>
  
  <p>NOTE: the image bitgandtter/sf:php7 use a VOLUME declaration on /var/www</p>