Make the temp mount as ready only in container WithVolumes

Signed-off-by: Qiutong Song <songqt01@gmail.com>
(cherry picked from commit ec90efb)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/cri/opts/container.go

@@ -66,6 +66,12 @@ func WithVolumes(volumeMounts map[string]string) containerd.NewContainerOpts {
 		if err != nil {
 			return err
 		}
+		// Since only read is needed, append ReadOnly mount option to prevent linux kernel
+		// from syncing whole filesystem in umount syscall.
+		if len(mounts) == 1 && mounts[0].Type == "overlay" {
+			mounts[0].Options = append(mounts[0].Options, "ro")
+		}
+
 		root, err := ioutil.TempDir("", "ctd-volume")
 		if err != nil {
 			return err