default gitlab.relative_url_root to ENV['RAILS_RELATIVE_URL_ROOT']

config/initializers/1_settings.rb

@@ -46,7 +46,7 @@ Settings.gitlab['default_projects_limit'] ||=  10
 Settings.gitlab['host']       ||= 'localhost'
 Settings.gitlab['https']        = false if Settings.gitlab['https'].nil?
 Settings.gitlab['port']       ||= Settings.gitlab.https ? 443 : 80
-Settings.gitlab['relative_url_root'] ||= ''
+Settings.gitlab['relative_url_root'] ||= ENV['RAILS_RELATIVE_URL_ROOT'] || ''
 Settings.gitlab['protocol']   ||= Settings.gitlab.https ? "https" : "http"
 Settings.gitlab['email_from'] ||= "gitlab@#{Settings.gitlab.host}"
 Settings.gitlab['support_email']  ||= Settings.gitlab.email_from