nix: share the service-container settings through one in-container module

The ten hand-rolled `containers.<name>` blocks each repeat the same
in-container lines: `system.stateVersion`, a firewall turned off because
the container shares the host netns, and resolvconf forced off because
something in the container writes /etc/resolv.conf itself.

`nix/host-modules/swarm-container.nix` now owns those lines. It is
imported inside the container's own config and exposes
`services.hyperhive.swarmContainer.{privateNetwork,writesOwnResolvConf}`
for the host module to set. `stateVersion` is a `mkDefault`, so the two
containers on another value can keep theirs. `--link-journal=host` stays
per module, and so do the host-side attrs (autoStart, ephemeral,
privateNetwork, bindMounts).

swarm-victoriametrics is converted as the first user. Its container
toplevel drvPath is unchanged. A module-eval case now forces that
container's config, which nothing in the suite read before.

Refs #3773

nix/host-modules/swarm-container.nix

@@ -0,0 +1,52 @@
+# What every hyperhive service nixos-container sets for itself, imported
+# inside the container's own `config`.
+#
+# The host module that declares `containers.<name>` sets the options below.
+# They restate host-side facts the container cannot read on its own: its
+# evaluation is nested inside `containers.<name>`, and reading the host side
+# back from in here recurses.
+{ config, lib, ... }:
+let
+  cfg = config.services.hyperhive.swarmContainer;
+in
+{
+  options.services.hyperhive.swarmContainer = {
+    privateNetwork = lib.mkOption {
+      type = lib.types.bool;
+      description = ''
+        Must equal the host-side `containers.<name>.privateNetwork`. When
+        `false` the container shares the host netns, so its own
+        firewall.service would rewrite the HOST ruleset at every boot; the
+        container's firewall is turned off and the host firewall owns all
+        filtering.
+      '';
+    };
+
+    writesOwnResolvConf = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = ''
+        Something in this container writes `/etc/resolv.conf` itself (the
+        `swarm-container-resolver.nix` unit, or a static file), so
+        resolvconf is forced off. Left on, host-tracking would regenerate the
+        file empty, since the host's copy doesn't cross the boundary after
+        start.
+      '';
+    };
+  };
+
+  config = lib.mkMerge [
+    {
+      # A container that sets its own keeps it. Changing this value changes
+      # it for every container that doesn't, and that is a state migration
+      # for each of them.
+      system.stateVersion = lib.mkDefault "26.05";
+    }
+    (lib.mkIf (!cfg.privateNetwork) {
+      networking.firewall.enable = false;
+    })
+    (lib.mkIf cfg.writesOwnResolvConf {
+      networking.resolvconf.enable = lib.mkForce false;
+    })
+  ];
+}

nix/host-modules/swarm-victoriametrics.nix

@@ -27,6 +27,10 @@ let
   # the required-domain assertion in hive-network.nix should be what an
   # operator sees, not a coercion error from here.
   domainBase = if swarmDomain == null then "invalid" else swarmDomain;
+
+  # Shared host netns, like every sibling swarm container: the gateway
+  # reaches this at 127.0.0.1:<port>.
+  privateNetwork = false;
 in
 {
   # What stays here is what the store IS from any hive's point of view: the
@@ -190,31 +194,20 @@ in
       # Journal files on the host, not inside the container: nixpkgs hardcodes
       # --link-journal=try-guest, and EXTRA_NSPAWN_FLAGS expands after it.
       extraFlags = [ "--link-journal=host" ];
-      # Shared host netns, like every sibling swarm container: the gateway
-      # reaches this at 127.0.0.1:<port>.
-      privateNetwork = false;
+      inherit privateNetwork;
 
       config =
         { ... }:
         {
           imports = [
+            ./swarm-container.nix
             (import ./swarm-container-resolver.nix {
               inherit (networkCfg) bridgeIp;
               dnsConsumers = [ "victoriametrics.service" ];
             })
           ];
 
-          system.stateVersion = "26.05";
-
-          # This container shares the host netns, so its own firewall.service
-          # would rewrite the HOST ruleset at every boot. The host firewall
-          # owns all filtering.
-          networking.firewall.enable = false;
-          # resolvconf stays off because the resolver unit imported above
-          # owns /etc/resolv.conf. Leaving it on would let host-tracking
-          # regenerate the file empty, since the host's copy doesn't cross
-          # the boundary after start.
-          networking.resolvconf.enable = lib.mkForce false;
+          services.hyperhive.swarmContainer = { inherit privateNetwork; };
 
           services.victoriametrics = {
             enable = true;

nix/module-eval/swarm-services-switch.nix

@@ -185,6 +185,21 @@ let
         && !(swarmServicesOnly.systemd.sockets ? hive-priv)
         && !(s ? swarm-bao-queue-agent);
     }
+    {
+      # Both values come from ../host-modules/swarm-container.nix. Read
+      # through the metrics store: nothing else in this suite evaluates that
+      # container's config.
+      name = "a service container on the host netns runs no firewall or resolvconf of its own";
+      ok =
+        let
+          c = swarmServicesOnly.containers.swarm-victoriametrics.config;
+        in
+        !c.networking.firewall.enable && !c.networking.resolvconf.enable;
+    }
+    {
+      name = "a service container that sets no stateVersion of its own is on 26.05";
+      ok = swarmServicesOnly.containers.swarm-victoriametrics.config.system.stateVersion == "26.05";
+    }
     {
       # An operator's explicit `false` beats every `mkDefault` assertion,
       # which is what keeps "asserted by whoever needs it" from being a