Fix sharing of the /dev tree to make sure we can safely umount the chroot when needed. This fixes local non-livefs-builder image builds.

live-build/functions

@@ -132,6 +132,7 @@ setup_mountpoint() {
     fi
 
     mount --rbind /dev "$mountpoint/dev"
+    mount --make-rslave "$mountpoint/dev"
     mount proc-live -t proc "$mountpoint/proc"
     mount sysfs-live -t sysfs "$mountpoint/sys"
     mount securityfs -t securityfs "$mountpoint/sys/kernel/security"