title: Jenkins pipeline: update image before docker.image().inside()
body: <p>I have a Jenkins pipeline described in a Jenkinsfile.</p>
  
  <p>During the build step I start a docker container in which all build steps are executed. Something like this:</p>
  
  <pre><code>docker.image('node:8').inside() { }
  </code></pre>
  
  <p>I notice that this does not fetch the latest docker image but always uses the image that was originally pulled from the docker hub. I was expecting that this would check with the docker hub if the <code>node:8</code> image is still up-to-date and fetch a new image if not. How can I trigger a new image download if my local registry does not have the latest image cached?</p>