packaging: move most of contrib/docker to contrib/packaging/docker
contrib/ is kind of a dumping ground for all kinds of random things.
Many of the files in contrib/ are related to packaging. But sorting
out how everything is related is a lot of effort since it is all
mixed together.
This commit starts the process of refactoring everything related
to packaging so it all lives under one roof.
Most content from contrib/docker is simply moved to
contrib/packaging/docker and all references are updated.
Differential Revision: https://phab.mercurial-scm.org/D3541
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).