ActionMailer provides a way to define the host to use with resource helpers:
config.action_mailer.default_url_options = { :host => "example.com" }
I can't find a way to force this to https.
All you have to do it add "protocol" to the default url options:
config.action_mailer.default_url_options = { :host => 'example.com', :protocol => 'https'}
Dazpinto was right - linking to a duplicate question with the answer: Rails mailer edit_user_url uses http not https