How to use cgroups within containers?

[Richard Weinberger]

Dear systemd and container folks,

at Plumbers the question raised how to provide cgroups to a systemd that lives
in a container (with user namespaces).
Due to the GDL train strikes I had to leave very soon and had no chance to
talk to you in person.

Was a solution proposed?
All I want to know is how to provide cgroups in a sane and secure way
to systemd. :-)

-- 
Thanks,
//richard