cgroups: Join groups by writing to cgroups.procs, not tasks

cgroups.procs moves all the threads of the process, and "tasks" just
the one thread. I believe there is a risk that we move the main thread,
but then we accidentally fork off one of the other threads if the go
scheduler randomly switched to another thread. So, it seems safer (and
more correct) to use cgroups.procs.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

pkg/cgroups/apply_raw.go

@@ -68,7 +68,7 @@ func (raw *rawCgroup) join(subsystem string, pid int) (string, error) {
 	if err := os.MkdirAll(path, 0755); err != nil && !os.IsExist(err) {
 		return "", err
 	}
-	if err := writeFile(path, "tasks", strconv.Itoa(pid)); err != nil {
+	if err := writeFile(path, "cgroup.procs", strconv.Itoa(pid)); err != nil {
 		return "", err
 	}
 	return path, nil