nixos-module/server/lxc-containers: add prebuilt containers

nix/lib/config/options.nix

@@ -138,6 +138,13 @@ let
   };
   hostOpts = { name, ... }: {
     options = {
+      prebuilt = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Include the container system in the server's `build-container` script.
+        '';
+      };
       role = mkOption {
         type = types.enum [ "ap" "switch" "server" "container" "client" ];
         default = "client";

nix/nixos-module/server/lxc-containers.nix

@@ -70,9 +70,27 @@ let
     mkdir -p /nix/var/nix/gcroots/lxc
 
     for c in $@; do
-      echo Building $c
-      nix build -o /nix/var/nix/gcroots/lxc/$c zentralwerk-network#$c-rootfs
-      SYSTEM=$(readlink /nix/var/nix/gcroots/lxc/$c)
+      unset SYSTEM
+
+      case "$c" in
+        ${builtins.concatStringsSep "\n" (
+          map (ctName: ''
+            ${ctName})
+              echo Using prebuilt system for container $c
+              SYSTEM=${self.packages.x86_64-linux."${ctName}-rootfs"}
+              ;;
+          '') (
+            builtins.attrNames (
+              lib.filterAttrs (_: { prebuilt, ... }: prebuilt)
+                containers
+            ))
+        )}
+        *)
+        echo Building $c
+        nix build -o /nix/var/nix/gcroots/lxc/$c zentralwerk-network#$c-rootfs
+        SYSTEM=$(readlink /nix/var/nix/gcroots/lxc/$c)
+          ;;
+      esac
 
       echo Installing $c
       for d in \