Removing Java 8

[basil]

Implementing Proposal A for adapting jenkinsci/docker to the removal of Java 8. Proposal A is not as elegant as Proposal B, so this PR is intended to be a fallback in case no implementation of Proposal B materializes over the coming month.

[dduportal]

@timja could you trigger the build of this PR? Since neither @basil or I are member of the repo, Jenkins uses the Jenkinsfile from the principal branch (message ‘Jenkinsfile’ has been modified in an untrusted revision in the build logs) so the changes proposed there are not picked up.

[timja]

@timja could you trigger the build of this PR? Since neither @basil or I are member of the repo, Jenkins uses the Jenkinsfile from the principal branch (message ‘Jenkinsfile’ has been modified in an untrusted revision in the build logs) so the changes proposed there are not picked up.

I couldn't see any way for me to trigger it and make it trusted.
I tried:

• pushing an empty commit
• modifying the Jenkinsfile
• clicking build now

Creating my own PR seemed to do it though:
#1357

[timja]

@basil / @dduportal also happy to grant write access if you'll be doing any more CI changes just let me know

e.g. if @dduportal you plan to try doing this via a flag in docker bake for proposal B