Docker 20.10.x

This is mostly a "note to self" -- I plan to work on this soon.

Docker used to release everything from https://github.com/docker/docker-ce -- this has changed with the latest Docker v20.10.x series.

Instead, Docker client is released from https://github.com/docker/cli, and the Docker engine is released from https://github.com/moby/moby/blob/master/vendor.conf. There may be some additional bits in other repos that we'll need to bring in.

I'm hopeful that we won't have to significantly alter aports/community/docker/APKBUILD or the resultant subpackages it builds, but this remains to be seen.