title: Jenkins pipeline/docker :Jenkins does not seem to be running inside a container
body: <p>I'm trying to execute the sample of code found in Jenkins Pipeline here : <a href="https://jenkins.io/doc/book/pipeline/docker/" rel="nofollow noreferrer">https://jenkins.io/doc/book/pipeline/docker/</a></p>
  <pre><code>node {
  /* Requires the Docker Pipeline plugin to be installed */
      docker.image('maven:3-alpine').inside('-v $HOME/.m2:/root/.m2') {
          stage('Build') {
              sh 'mvn -B'
          }
      }
  }
  </code></pre>
  <p>And give me this error:</p>
  <pre><code>[Pipeline] withDockerContainer
  Jenkins does not seem to be running inside a container 
  [Pipeline] // withDockerContainer
  </code></pre>
  <p>I don't know why is it stopping like that without doing anything.</p>
  <p>I have already install docker, docker plugin/docker pipeline on the latest version.</p>
  <p>In configuration tool, i add the installation root path.</p>
  <p>Did I miss something ?
  Thanks in advance</p>