semi-spread vm test: the merged swarm machine keeps attic private, like heavyduty

nix/vm-tests/semi-spread.nix

@@ -224,6 +224,9 @@ pkgs.testers.runNixOSTest (
           # The test pushes its own hyperhive/hyperhive; a mirror of the
           # upstream could not be created offline and would race the push.
           deploy.forgejo.hyperhiveUpstream = null;
+          # The cache answers on the private addresses only, as on the
+          # example's heavyduty.
+          gateway.vhosts.${config.services.hyperhive.swarm.attic.ownDomain}.public = false;
         };
 
         # 8 GB: the store and the swarm's services in containers, the