title: Jenkins Docker container not copying pre-installed plugins to JENKINS_HOME on start
body: <p>I cannot seem to get Jenkins to copy pre-installed plugins to the expected <code>/var/jenkins_home/plugins</code> directory on startup as documented <a href="https://github.com/jenkinsci/docker#preinstalling-plugins" rel="nofollow noreferrer">here</a>. </p>
  
  <p>I'm using a very basic Dockerfile:</p>
  
  <pre><code>FROM jenkins:alpine
  USER root
  RUN /usr/local/bin/install-plugins.sh ssh-agent
  USER jenkins
  </code></pre>
  
  <p>When I build the container, it says the plugins installed successfully and after I run the container:</p>
  
  <ul>
  <li>the plugins do not appear in the web console and restarting does not
  fix the issue. </li>
  <li><code>/var/jenkins_home/plugins</code> inside the container is empty</li>
  <li><code>/usr/share/jenkins/ref/plugins</code> does not exist inside the container</li>
  </ul>
  
  <p><hr>
  <strong>However</strong> I do see the expected plugins inside of 
  <code>/var/jenkins_home/war/WEB-INF/detached-plugins</code> </p>
  
  <p>I attempted to copy them by adding:</p>
  
  <pre><code>RUN cp -R /var/jenkins_home/war/WEB-INF/detached-plugins/* /var/jenkins_home/plugins/`
  </code></pre>
  
  <p>but then I get the following error during build:</p>
  
  <pre><code>cp: cannot stat '/var/jenkins_home/war/WEB-INF/detached-plugins/*': No such file or directory`
  </code></pre>