switch xcache.nspawn config to use volatile containers with overlayfs

etc/systemd/nspawn/xcache.nspawn

@@ -5,7 +5,14 @@ Bind=/var/run/xrootd
 BindReadOnly=/etc/grid-security
 BindReadOnly=/etc/xrootd
 
-PrivateUsersChown=True
+# Mount root file system in container as overlayfs where writes go to a tmpfs
+# such that the image directory will not be overwritten by running containers
+#
+# before this can work we need to run once before with
+# --private-users-chown
+# e.g.
+# systemd-nspawn --volatile=no --private-users-chown --private-users=655360 -D /var/lib/machines/xcache ls -la /
+Volatile=overlay
 
 [Exec]
 #PrivateUsers=False
@@ -33,12 +40,5 @@ Environment=X509_USER_PROXY=/var/run/xrootd/x509up
 Environment=XRD_METALINKPROCESSING=1
 Environment=XRD_LOCALMETALINKFILE=1
 
-# store journal on host system and bind mount into container
-LinkJournal=host
-
-# Enable this to always make temporary containers (don't write back to the image)
-#Ephemeral=True
-#LinkJournal=False
-
 [Network]
 VirtualEthernet=False