duplicati: allow changing the user

nixos/modules/services/backup/duplicati.nix

@@ -26,6 +26,15 @@ in
           Set it to "any" to listen on all available interfaces
         '';
       };
+
+      user = mkOption {
+        default = "duplicati";
+        type = types.str;
+        description = ''
+          Duplicati runs as it's own user. It will only be able to backup world-readable files.
+          Run as root with special care.
+        '';
+      };
     };
   };
 
@@ -37,14 +46,19 @@ in
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
       serviceConfig = {
-        User = "duplicati";
+        User = cfg.user;
+        PermissionsStartOnly = true;
         Group = "duplicati";
         ExecStart = "${pkgs.duplicati}/bin/duplicati-server --webservice-interface=${cfg.interface} --webservice-port=${toString cfg.port} --server-datafolder=/var/lib/duplicati";
         Restart = "on-failure";
       };
+      preStart = ''
+        mkdir -p /var/lib/duplicati
+        chown -R ${cfg.user}:duplicati /var/lib/duplicati
+      '';
     };
 
-    users.users.duplicati = {
+    users.users.duplicati = lib.optionalAttrs (cfg.user == "duplicati") {
       uid = config.ids.uids.duplicati;
       home = "/var/lib/duplicati";
       createHome = true;