Bug#943981: Proposal: Switch to cgroupv2 by default

[El boulangero]

Hello all, here comes news from the docker package.

I can confirm, if ever there was a need, that docker 19.03 does not work
with `systemd.unified_cgroup_hierarchy=true`.

  $ dpkg -l | grep docker
  ii  docker.io  19.03.13+dfsg3-2  amd64  Linux container runtime

  $ findmnt /sys/fs/cgroup
  TARGET         SOURCE  FSTYPE  OPTIONS
  /sys/fs/cgroup cgroup2 cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate

  $ systemctl is-active docker
  active

  $ sudo docker run --rm -it debian
  docker: Error response from daemon: cgroups: cgroup mountpoint does not
exist: unknown.

Docker will support cgroupv2 in the upcoming `20.10` version. They released
a rc1 a few days ago:
- <https://github.com/moby/moby/releases/tag/v20.10.0-rc1>

I packaged this version and I just uploaded it to experimental:
- <https://salsa.debian.org/docker-team/docker/-/tree/experimental>
- <
https://buildd.debian.org/status/package.php?p=docker.io&suite=experimental>

I can confirm with a quick test that it seems to work:

  $ dpkg -l | grep docker
  ii  docker.io  20.10.0~rc1+dfsg1-1  amd64  Linux container runtime

  $ sudo docker run --rm -it debian echo 'hello world'
  hello world

Please note that the package I uploaded to experimental is a work in
progress. Please give it a try and report issues.

Whether Docker upstream will release a "stable" version in time for Debian
Bullseye is another question. Wait and see.

Cheers,

  Arnaud
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