add support for cgroupns

[christf]

SUMMARY

Some containers ONLYOFFICE/Docker-CommunityServer#131 - apparently those that run systemd - will require --cgroupns=host to be set.
This is currently not possible with the docker role. Please consider adding the fature

ISSUE TYPE

• Feature Idea

COMPONENT NAME

ADDITIONAL INFORMATION

being able to set

cgroupns: host 

woulrd be very useful to run this kind of container.

[felixfontein]

--cgroupns corresponds to the CgroupnsMode host config (judging from the docker-cli sources), but Docker SDK for Python does not (yet) support that.

(And judging the state of the Docker SDK for Python project, I wouldn't hope that it will support it anytime soon... :( )

[felixfontein]

(Also there is no role in this collection. We only have modules and plugins.)

[felixfontein]

I created an implementation of this in #427 (based on the rewrite #422 of docker_container to use the API directly).