Configure additional rails hosts with env variable

changelogs/unreleased/configure-additional-rails-hosts-with-env-variable.yml

+---
+title: Add possibility to conigure additional rails hosts with env variable
+merge_request: 28133
+author:
+type: other

config/initializers/rails_host_authorization.rb

@@ -4,4 +4,9 @@
 
 if Rails.env.development?
   Rails.application.config.hosts += [Gitlab.config.gitlab.host, 'unix']
+
+  if ENV['RAILS_HOSTS']
+    additional_hosts = ENV['RAILS_HOSTS'].split(',').select(&:presence)
+    Rails.application.config.hosts += additional_hosts
+  end
 end