Add mount point to cgroup root when initializing cgroup paths for

cpu.rt_runtime

PR https://github.com/docker/docker/pull/23430 introduced a couple more
flags including `--cpu-rt-runtime` to the docker daemon. It appears
recent changes or merge issues may have broken this. It currently does
not take the cgroup mount point into account when determining the cgroup
files to write values to. This breaks docker setting its own
`cpu.rt_runtime` for the daemon. This also means containers aren't able
to set theirs.

Also, the cgroups.FindCgroupMountpointAndRoot returns back a mount point
that includes the cgroup of the currently running container when docker
is run inside a docker container. this breaks the `--cpu-rt-runtime`
flag when running docker in docker. A fix has been placed here, but
potentially could be pulled up into libcontainer if this is a better
place for it.

Signed-off-by: Erik St. Martin <alakriti@gmail.com>

daemon/daemon_unix.go

@@ -1269,12 +1269,17 @@ func (daemon *Daemon) initCgroupsPath(path string) error {
 	// for the period and runtime as this limits what the children can be set to.
 	daemon.initCgroupsPath(filepath.Dir(path))
 
-	_, root, err := cgroups.FindCgroupMountpointAndRoot("cpu")
+	mnt, root, err := cgroups.FindCgroupMountpointAndRoot("cpu")
 	if err != nil {
 		return err
 	}
+	// When docker is run inside docker, the root is based of the host cgroup.
+	// Should this be handled in runc/libcontainer/cgroups ?
+	if strings.HasPrefix(root, "/docker/") {
+		root = "/"
+	}
 
-	path = filepath.Join(root, path)
+	path = filepath.Join(mnt, root, path)
 	sysinfo := sysinfo.New(true)
 	if err := maybeCreateCPURealTimeFile(sysinfo.CPURealtimePeriod, daemon.configStore.CPURealtimePeriod, "cpu.rt_period_us", path); err != nil {
 		return err