Make Action Mailer use request host and protocol for images and links in emails.

application_controller.rb

# application_controller.rb
class ApplicationController < ActionController::Base

  before_action :make_action_mailer_use_request_host_and_protocol
  
  def make_action_mailer_use_request_host_and_protocol
    ActionMailer::Base.default_url_options[:protocol] = request.protocol
    ActionMailer::Base.default_url_options[:host] = request.host_with_port
    ActionMailer::Base.asset_host = "#{request.protocol}#{request.host_with_port}"
  end
end