fallback to rails default when '/' root supplied

config/environments/production.rb

@@ -88,5 +88,7 @@ Rails.application.configure do
   # Do not dump schema after migrations.
   config.active_record.dump_schema_after_migration = false
 
-  config.relative_url_root = ENV['RAILS_RELATIVE_URL_ROOT'] || '/b'
+  if ENV['RELATIVE_URL_ROOT'] != '/'
+    config.relative_url_root = ENV['RELATIVE_URL_ROOT'] || '/b'
+  end
 end

env

@@ -67,6 +67,6 @@ GOOGLE_OAUTH2_SECRET=
 # Prefix for the application's root URL
 # Useful for deploying the application to a subdirectory
 #
-#   default: '/b' (recommended)
+#   default is '/b' (recommended)
 #
-#RAILS_RELATIVE_URL_ROOT=/b
+#RELATIVE_URL_ROOT=/b