Allow joining cgroups of another container

[mrunalp]

The spec allows specifying namespace fds via paths and that feature allows one container to share the namespaces of another container. Another feature to make that more useful is allowing a container to join the cgroups of another container using its pid. This will make it possible for higher level tooling to add nsinit/nsenter like functionality.

@crosbymichael @philips WDYT?

[philips]

Why not use the cgroup name?

[mrunalp]

@philips That works, too. I think there should be a way to solve the use case.

[philips]

@mrunalp Wait, sorry, what information would you add on top of the kernel metadata? e.g. /proc/571/cgroup

[wking]

On Tue, Jul 28, 2015 at 12:05:54PM -0700, Brandon Philips wrote:

@mrunalp Wait, sorry, what information would you add on top of the
kernel metadata? e.g. /proc/571/cgroup

In the absence of further information here, I think we can close this
issue once #137 lands.

[mrunalp]

Yes, we can close this with #137.

[wking]

#137 has landed. Close?