manager: don't do plymouth in a container

Given that plymouth listens on an abstract namespace socket and if
CLONE_NEWNET is not used the abstract namespace is shared with the host
we might actually end up send plymouth data to the host.

src/core/manager.c

@@ -1806,6 +1806,9 @@ void manager_send_unit_plymouth(Manager *m, Unit *u) {
         if (m->running_as != SYSTEMD_SYSTEM)
                 return;
 
+        if (detect_container(NULL) > 0)
+                return;
+
         if (u->type != UNIT_SERVICE &&
             u->type != UNIT_MOUNT &&
             u->type != UNIT_SWAP)