Dynamic subject for Devise mailers (i.e. reset_password_instructions)

custom_devise_mailer.rb

class CustomDeviseMailer < Devise::Mailer
  helper :application # gives access to all helpers defined within `application_helper`.
  include Devise::Controllers::UrlHelpers # Optional. eg. `confirmation_url`
  default template_path: 'devise/mailer' # to make sure that your mailer uses the devise views

  def reset_password_instructions(record, token, opts={})
    opts[:subject] = I18n.t(:'devise.mailer.reset_password_instructions.subject', app_name: ENV['APP_NAME'])
  	super
  end
end

devise.en.yml

en:
  devise:
  .......
  .......
  mailer:
      confirmation_instructions:
        subject: "Confirmation instructions"
      reset_password_instructions:
        subject: "Reset your %{app_name} Password"
      unlock_instructions:
        subject: "Unlock instructions"
      password_change:
        subject: "Your %{app_name} Password Changed"
  .......
  .......