title: Can a Dockerfile extend another one?
body: <p>I have a Dockerfile for PHP like this :</p>
  
  <pre><code>FROM php:7-fpm
  ENV DEBIAN_FRONTEND noninteractive
  
  RUN apt-get update &amp;&amp; \
      apt-get install -y git libicu-dev libmagickwand-dev libmcrypt-dev libcurl3-dev jpegoptim
  RUN pecl install imagick &amp;&amp; \
      docker-php-ext-enable imagick
  
  RUN docker-php-ext-install intl
  RUN docker-php-ext-install pdo_mysql
  RUN docker-php-ext-install opcache
  RUN docker-php-ext-install mcrypt
  RUN docker-php-ext-install curl
  RUN docker-php-ext-install zip
  </code></pre>
  
  <p>And I'd like to create another Dockerfile, based on the first one, but with some PHP extensions added (for dev purpose) : Xdebug and other stuffs.</p>
  
  <p>Can I create a "dev" Dockerfile that extends my main Dockerfile (without rewrite it) ?</p>