title: Configuring a Jenkins pipeline to ignore building unchanged projects
body: <p>I have a Jenkins pipeline that is triggered by a bitbucket webhook every time a pull request is created, and builds docker images.
  My bitbucket repository contains multiple projects, each with it's own subfolder and Dockerfile.
  I wouldn't want the pipeline to build docker images for all projects every time it is triggered.
  How do I tell the pipeline to only build docker images for those projects that have changed?</p>
  
  <p>Thanks,
  O.</p>