Allow :site_url to be picked up from ENV vars

config/initializers/spree.rb

@@ -18,6 +18,7 @@ Spree::Gateway.class_eval do
 end
 
 Spree.config do |config|
+  config.site_url = ENV['SITE_URL'] if ENV['SITE_URL']
   config.shipping_instructions = true
   config.address_requires_state = true
   config.admin_interface_logo = '/default_images/ofn-logo.png'

lib/spree/core/mail_settings.rb

@@ -12,7 +12,7 @@ module Spree
       end
 
       def override!
-        ActionMailer::Base.default_url_options[:host] ||= Config.site_url
+        ActionMailer::Base.default_url_options[:host] ||= ENV.fetch("SITE_URL", Config.site_url)
         ActionMailer::Base.smtp_settings = mail_server_settings
         ActionMailer::Base.perform_deliveries = true
       end