Add docker gradle plugin defunct declaration

[xRuiAlves]

Before this PR

There's no info on the repo stating that the docker plugin is no longer supported and should no longer be used.

After this PR

==COMMIT_MSG== The docker plugin is declared as defunct. ==COMMIT_MSG==

Possible downsides?

[changelog-app[bot]]

Generate changelog in changelog/@unreleased

Type See change types. Select one:

• [ ] Feature
• [x] Improvement
• [ ] Fix
• [ ] Break
• [ ] Deprecation
• [ ] Manual task
• [ ] Migration

Description The docker plugin is declared as defunct.

Check the box to generate changelog(s)

• [x] Generate changelog entry

[tpetracca]

Do expect folks to still use docker-compose and docker-run plugins and have them work? Or is real goal that this entire repo is dead? CC @CRogers

[xRuiAlves]

Cool, thanks @carterkozak! 👍

[xRuiAlves]

Oops, had forgotten about this one! Un-assigning myself :)

[CRogers]

:+1: