virtualisation/digital-ocean: use system.build.image

nixos/modules/virtualisation/digital-ocean-image.nix

@@ -14,6 +14,7 @@ in
   imports = [
     ./digital-ocean-config.nix
     ./disk-size-option.nix
+    ../image/file-options.nix
     (lib.mkRenamedOptionModuleWith {
       sinceRelease = 2411;
       from = [
@@ -57,10 +58,33 @@ in
   };
 
   #### implementation
-  config = {
+  config =
+    let
+      format = "qcow2";
+    in
+    {
+      image.extension = lib.concatStringsSep "." [
+        format
+        (
+          {
+            "gzip" = "gz";
+            "bzip2" = "bz2";
+          }
+          .${cfg.compressionMethod}
+        )
+      ];
+      system.nixos.tags = [ "digital-ocean" ];
+      system.build.image = config.system.build.digitalOceanImage;
       system.build.digitalOceanImage = import ../../lib/make-disk-image.nix {
         name = "digital-ocean-image";
-      format = "qcow2";
+        inherit (config.image) baseName;
+        inherit (config.virtualisation) diskSize;
+        inherit
+          config
+          lib
+          pkgs
+          format
+          ;
         postVM =
           let
             compress =
@@ -78,8 +102,6 @@ in
             config.virtualisation.digitalOcean.defaultConfigFile
           else
             cfg.configFile;
-      inherit (config.virtualisation) diskSize;
-      inherit config lib pkgs;
       };
 
     };