Missing host

gistfile1.rb

# rails (4.2.0), devise (3.5.1)

# in config/environments/staging.rb
config.action_mailer.default_url_options = { host: ENV['HOSTNAME'] }
Rails.application.routes.default_url_options[:host] = ENV['HOSTNAME']
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  address: ENV['SMTP_SERVER'],
  port: ENV['SMTP_PORT'],
  user_name: ENV['SMTP_USER'],
  password: ENV['SMTP_PASSWORD'],
  authentication: :plain,
  enable_starttls_auto: true
}

# staging.log

Devise::Mailer#confirmation_instructions: processed outbound mail in 6.5ms
D, [2015-05-28T03:42:08.523456 #3660] DEBUG -- :    (2.2ms)  ROLLBACK
I, [2015-05-28T03:42:08.528776 #3660]  INFO -- : Completed 500 Internal Server Error in 127ms
F, [2015-05-28T03:42:08.533899 #3660] FATAL -- : 
ActionView::Template::Error (Missing host to link to! Please provide the :host parameter, set default_url_options[:host], or set :only_path to true):
    2: 
    3: <p>You can confirm your account email through the link below:</p>
    4: 
    5: <p><%= link_to 'Confirm my account', confirmation_url(@resource, confirmation_token: @token) %></p>
  actionpack (4.2.0) lib/action_dispatch/http/url.rb:45:in `full_url_for'