docker run flag for overriding daemon default-cgroupns-mode

[AkihiroSuda]

The daemon now supports --default-cgroupns-mode=(host|private): moby/moby#38377

Some candidates for docker run flag name:

• --cgroupns-mode=(host|private): consistent with dockerd --default-cgroupns-mode=(host|private))
• --cgroupns=(host|private) : consistent with docker run --userns=(host|"")
• --cgroup=(host|private): consistent with docker run --uts=(host|"")

@thaJeztah @rgulewich

[thaJeztah]

Wondering if --cgroup may be confusing (it's not really clear that it sets the namespace for cgroups). One more option could be --cgroup-mode=(host|private) (without ns)

@tiborvass @silvin-lubecki wdyt?

[rgulewich]

@AkihiroSuda / @thaJeztah / @tiborvass @silvin-lubecki - took a quick swing at --cgroupns here: #2024

Please give it a look!