title: ssh-keyscan not found in Dockerfile
body: <p>I have a simple Dockerfile, exactly as below:</p>
  
  <pre><code>FROM ubuntu:14.04
  
  RUN apt-get update
  RUN apt-get -y upgrade
  
  RUN mkdir -p /root/.ssh
  RUN touch /root/.ssh/known_hosts
  
  RUN ssh-keyscan github.com &gt;&gt; /root/.ssh/known_hosts
  </code></pre>
  
  <p>The result of running:</p>
  
  <pre><code>docker build -no-cache -t testimage .
  </code></pre>
  
  <p>is:</p>
  
  <pre><code>Step 5 : RUN ssh-keyscan github.com &gt;&gt; /root/.ssh/known_hosts
   ---&gt; Running in e11ef5962a11
  /bin/sh: 1: ssh-keyscan: not found
  </code></pre>