title: Jenkins unable to install plugin unknownhost
body: <p>I have installed Oracle VM and created a centos 7 machine
  in which I have installed Docker successfully</p>
  <p>then I installed jenkins from docker image : jenkins/jenkins</p>
  <p>in that I am not able to install plugins, it is giving me below error:
  java.net.UnknownHostException: mirrors.tuna.tsinghua.edu.cn</p>
  <p>I have reinstalled couple of times but the problem still exists.</p>
  <p>I tried to ping google in jenkins container and that is working fine.</p>
  <p>Please help me out here.</p>