nixos/gitlab: add extraEnv option

This allows users to define custom environment variables for gitlab, 
without having to modify the service file directly

nixos/modules/services/misc/gitlab.nix

@@ -156,7 +156,7 @@ let
     prometheus_multiproc_dir = "/run/gitlab";
     RAILS_ENV = "production";
     MALLOC_ARENA_MAX = "2";
-  };
+  } // cfg.extraEnv;
 
   gitlab-rake = pkgs.stdenv.mkDerivation {
     name = "gitlab-rake";
@@ -277,6 +277,14 @@ in {
         '';
       };
 
+      extraEnv = mkOption {
+        type = types.attrsOf types.str;
+        default = {};
+        description = ''
+          Additional environment variables for the GitLab environment.
+        '';
+      };
+
       backup.startAt = mkOption {
         type = with types; either str (listOf str);
         default = [];