Merge pull request #13436 from NixOS/mergify/bp/2.30-maintenance/pr-13435

docker: fix nixConf (backport #13435)

Author: mergify[bot]

docker.nix

@@ -184,11 +184,14 @@ let
       } " = ";
     };
 
-  nixConfContents = toConf {
-    sandbox = false;
-    build-users-group = "nixbld";
-    trusted-public-keys = [ "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" ];
-  };
+  nixConfContents = toConf (
+    {
+      sandbox = false;
+      build-users-group = "nixbld";
+      trusted-public-keys = [ "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" ];
+    }
+    // nixConf
+  );
 
   userHome = if uid == 0 then "/root" else "/home/${uname}";