A new method was added to the cgroup interface when #2177 was merged.
After #2177 got merged, #2169 was merged without rebase (sorry!) and compilation was failing:
libcontainer/cgroups/fs2/fs2.go:208:22: container.Cgroup undefined (type *configs.Config has no field or method Cgroup)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>