title: Docker System has not been booted with systemd as init system
body: <p>I have an Ubuntu 18.04 image runing on my docker container. I login into it and installed <code>Openresty</code>. also installed <code>systemd</code>. When I use command <code>systemctl</code> I get this error:</p>
  <pre><code>System has not been booted with systemd as init system (PID 1). Can't operate.
  </code></pre>
  <p>How can I fix it?</p>