Usually GUI web apps don’t like to be placed on a path, they expect to be root. It only works when some kind of "base path" can be set within the application.
The reason is that html pages include links and load further dependencies. Those are usually with absolute paths. The initial page will load with /docmost using PathPrefix() and stripPrefix, but included /static/script.js request will not be matched by router rule and never reach the correct target service.
So this is a specific application question, you probably find an answer easier in a dedicated application forum.