Prepare for sunset icon removal from core

[NotMyFault]

This is a preparation for jenkinsci/jenkins#5778 which removes sunset icons.

Otherwise we're lacking an icon here:

Which has been mitigated:

cc @pjdarton

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[pjdarton]

The automated tests have failed 3 times in a row but that seems to be a jenkinsci infrastructure issue rather than anything to do with the code ...
I'll take this one "on trust" and merge it.

...is there any rush to push out a new release, or is that some way down the line e.g. next year?

[NotMyFault]

The core PR I mentioned is on hold until some other PRs are merged, like this one. Depending on when it's merged, it may be part of the next LTS release but definitely will be part of any weekly release in the sooner future.
timja has asked authors to draft a release for it e.g. in https://github.com/jenkinsci/pipeline-stage-view-plugin/pull/182#issuecomment-991922805 but I'm not sure if there's a general policy for it.

But in my opinion it would be handy to have plugins to drop releases before core updates, to prevent missing icons in the UI.

[pjdarton]

Released as docker-plugin-1.2.6.hpi