Update hack/dind to mount cgroups on "/cgroup" instead of "/sys/fs/cgroup" for better compatibility

Fixes #5122

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #5463 from tianon/hack-dind-cgroup

Update hack/dind to mount cgroups on "/cgroup" instead of "/sys/fs/cgroup" for better compatibility