dockerd-rootless.sh: support containerd v1.4 shim socket path convention

The new shim socket path convention hardcodes `/run/containerd`:
https://github.com/containerd/containerd/pull/4343

`dockerd-rootless.sh` is updated to hide the rootful `/run/containerd`
from the mount namespace of the rootless dockerd.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit 794aa20983c9384726721e1c5d3a552ef7f8f4cf)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

contrib/dockerd-rootless.sh

@@ -88,6 +88,6 @@ else
 	[ $_DOCKERD_ROOTLESS_CHILD = 1 ]
 	# remove the symlinks for the existing files in the parent namespace if any,
 	# so that we can create our own files in our mount namespace.
-	rm -f /run/docker /run/xtables.lock
+	rm -f /run/docker /run/containerd /run/xtables.lock
 	exec dockerd $@
 fi