title: Docker build: read-only file system
body: <p>I'm using a <code>Dockerfile</code> to build my image and I have a command in there that says:</p>
  
  <pre><code>RUN sysctl -w net.ipv4.route.flush=1
  </code></pre>
  
  <p>but it fails to build the image with the following error:</p>
  
  <pre><code>Step 20 : RUN sysctl -w net.ipv4.route.flush=1
   ---&gt; Running in 4d7302b56c53
  sysctl: setting key "net.ipv4.route.flush": Read-only file system
  </code></pre>