Your nginx looks fine to me.
In your Go code, when you create your router, you may use the myapp as the PathPrefix like below:
baseRouter := mux.NewRouter()
subRouter := baseRouter.PathPrefix("/myapp").Subrouter()
subRouter.HandleFunc("/something", routes.SomeHandler).Methods("GET")
Or simply add myapp to the path: baseRouter.HandleFunc("/myapp/something", routes.SomeHandler).Methods("GET")