Enter the container and edit /etc/nginx/conf.d/discourse.conf to have it serve files from /sitemap. You can Google nginx for help on how you might do that.
You can get nginx to reload the config with
sv restart nginx
That will solve your immediate problem. The you can go about making changes to app.yml that will make those changes when you rebuild the container.
If that doesn’t make sense and it’s urgent and you have a budget my contact info is in my profile.