Added cashew, did some backplane yak shaving

lib/instance.nix

@@ -12,6 +12,11 @@ in {
       description = "Hostname of this specific host (without domain).";
     };
 
+    host-fqdn = mkOption {
+      type = str;
+      description = "Fully-qualified name of this host.";
+    };
+
     build-timestamp = mkOption {
       type = int;
       description = "Timestamp associated with the build. Used for e.g. DNS serials.";
@@ -98,9 +103,12 @@ in {
 
     local-profile = host.profile;
 
+    host-fqdn = "${config.instance.hostname}.${local-domain}";
+
   in {
     instance = {
       inherit
+        host-fqdn
         local-domain
         local-site
         local-users