If xdg-user-dirs-update has never run on the host, then xdg-app run bails out with the following error:

  "Can't find source path /home/builder/.config/user-dirs.dirs: No such file or directory"

The following change in (pre-flatpak) common/xdg-app-run.c:xdg_app_run_add_environment_args() seems to address the issue:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--- a/common/xdg-app-run.c
+++ b/common/xdg-app-run.c
@@ -1785,9 +1785,11 @@ xdg_app_run_add_environment_args (GPtrArray     *argv_array,
                                                     NULL);
       g_autofree char *path = g_build_filename (gs_file_get_path_cached (app_id_dir),
                                                 "config/user-dirs.dirs", NULL);
-      add_args (argv_array,
-                "--ro-bind", src_path, path,
-                NULL);
+
+      if (g_file_test (src_path, G_FILE_TEST_EXISTS))
+       add_args (argv_array,
+                 "--ro-bind", src_path, path,
+                 NULL);
     }
   else if (xdg_dirs_conf != NULL && app_id_dir != NULL)
     {
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

But I cant say if that is correct, perhaps a more informative error message would be nice if the user-dirs.dirs is expected in the runtime environment.