CLDC-2812: update app config to run from relative root

config.ru

@@ -2,5 +2,7 @@
 
 require_relative "config/environment"
 
-run Rails.application
-Rails.application.load_server
+map DataCollector::Application.config.relative_url_root || "/" do
+  run Rails.application
+  Rails.application.load_server
+end

config/application.rb

@@ -50,5 +50,7 @@ module DataCollector
                        helper_specs: false,
                        controller_specs: false
     end
+
+    config.relative_url_root = ENV["RAILS_RELATIVE_URL_ROOT"]
   end
 end