title: Pass empty string (double quote) argument to dockerfile
body: <p>My project wrapped by Docker and run in 2 environments: <strong>behind proxy</strong> or <strong>no proxy</strong>.</p>
  <p>In this case, I use <em>php:apache</em> image and install ext via pecl, so I must set proxy manually</p>
  <pre><code>FROM php:apache
  
  RUN pear config-set http_proxy $http_proxy
  # RUN pecl install mongodb &amp;&amp; docker-php-ext-enable mongodb
  </code></pre>
  <p>The <code>$http_proxy</code> arg will passed from docker-compose in 2 values: <code>http://server:port</code> or <code>“”</code> (double quote)</p>
  <p>But when build the custom image in case <code>“”</code>, the <code>$http_proxy</code> arg was null, and show error in the config command</p>
  <blockquote>
  <p>Step 4/4 : RUN pear config-set http_proxy $http_proxy</p>
  <p>—&gt; Running in 19b69d089ff2</p>
  <p>config-set expects 2 or 3 parameters</p>
  </blockquote>