title: if else clause for Dockerfile variable not working
body: <p>The following doesn't work in my <code>Dockerfile</code>:</p>
  
  <pre><code>RUN if $SSH_PRIVATE_KEY -eq ""; then echo "SSH key is not set, aborting"; exit 1; else echo "SSH key is set"; fi
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>Step 4/15 : RUN if $SSH_PRIVATE_KEY -eq ""; then echo "SSH key is not set, aborting"; exit 1; else echo "SSH key is set"; fi
   ---&gt; Running in 3bd29320f0e3
  /bin/sh: -eq: command not found
  </code></pre>
  
  <p>I would like to check if the <code>$SSH_PRIVATE_KEY</code> variable contains anything at all.</p>