Disable container support in containers

Systemd-nspawn doesn't support nesting, so providing nixos-container
inside a container doesn't make sense.

nixos/modules/virtualisation/containers.nix

@@ -140,7 +140,7 @@ in
   };
 
 
-  config = {
+  config = mkIf (!config.boot.isContainer) {
 
     systemd.services."container@" =
       { description = "Container '%i'";
@@ -222,7 +222,8 @@ in
         postStart =
           ''
             # This blocks until the container-startup-done service
-            # writes something to this pipe.
+            # writes something to this pipe.  FIXME: it also hangs
+            # until the start timeout expires if systemd-nspawn exits.
             read x < $root/var/lib/startup-done
           '';