jenkins: add documentation for update center

[bowling233]

The layout of Tuna's Jenkins image differs from the official one, and it's easy to make mistakes when configuring it in the Dockerfile. Additionally, there is limited documentation available for this configuration. Hopefully, this will help those in need.

[Harry-Chen]

What about the layout of other mirrors?

[bowling233]

Sorry for the confusion, I misunderstood the Jenkins mirror setup. There are actually three official sites: updates.jenkins.io, pkg.jenkins.io, and mirrors.jenkins-ci.org.

• mirrors.jenkins-ci.org combines content from both updates.jenkins.io and pkg.jenkins.io. It retains the layout of pkg.jenkins.io, but the content from updates.jenkins.io is nested under an updates folder. This setup can be confusing for new users.
• All Jenkins mirrors follow the same layout as mirrors.jenkins-ci.org, which differs from updates.jenkins.io, the source used for the JENKINS_UC environment variable in the Jenkins Dockerfile. This means that the environment variable setup needs to be adjusted accordingly in the Dockerfile for all mirror user.

This issue is not specific to TUNA, and I'll update my pull request accordingly.