google-compute-image: Make GZIP compression level configurable

Change-Id: I1bde130f0a951fa40b1a16bad09497c796bca473

nixos/modules/virtualisation/google-compute-image.nix

@@ -36,6 +36,14 @@ in
         `<nixpkgs/nixos/modules/virtualisation/google-compute-image.nix>`.
       '';
     };
+
+    virtualisation.googleComputeImage.compressionLevel = mkOption {
+      type = types.int;
+      default = 6;
+      description = ''
+        GZIP compression level of the resulting disk image (1-9).
+      '';
+    };
   };
 
   #### implementation
@@ -47,7 +55,8 @@ in
         PATH=$PATH:${with pkgs; lib.makeBinPath [ gnutar gzip ]}
         pushd $out
         mv $diskImage disk.raw
-        tar -Szcf nixos-image-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.raw.tar.gz disk.raw
+        tar -Sc disk.raw | gzip -${toString cfg.compressionLevel} > \
+          nixos-image-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.raw.tar.gz
         rm $out/disk.raw
         popd
       '';