jenkins: capture a stable starting point for jenkins images
jenkins/jenkins:lts is an inadequate starting point for ensuring reproducible docker image builds. While the lts tag is stable, we are not in control of when it gets updated.
We need to be able to build a new jenkins container, to add a package (for example) with NO OTHER changes made to the deployed container. For this to work, we must have recorded a stable starting point.
At the moment, jenkins/jenkins:lts is a reference to the 2.440.3-lts-jdk17 tag, which will not change. It was pulled and saved on our internal registry.