support relative root urls for deploy to subdirectories

app/controllers/application_controller.rb

@@ -29,4 +29,9 @@ class ApplicationController < ActionController::Base
     @current_user ||= User.find_by(id: session[:user_id])
   end
   helper_method :current_user
+
+  def relative_root
+    Rails.configuration.relative_url_root || ""
+  end
+  helper_method :relative_root
 end

app/controllers/bbb_controller.rb

@@ -66,7 +66,7 @@ class BbbController < ApplicationController
         }
       end
 
-      base_url = "#{request.base_url}/#{params[:resource]}/#{meeting_path}"
+      base_url = "#{request.base_url}#{relative_root}/#{params[:resource]}/#{meeting_path}"
       options[:meeting_logout_url] = base_url
       options[:hook_url] = "#{base_url}/callback"