Currently, the option "Custom environment variables" in the menu "Ruby" on a domain is used only for passing environment variables to the application.
But it's also required to pass environment variables to the rake tasks as well, and in the current implementation, it's necessary to specify necessary variables in the window "Run rake task" each time a rake tasks needs to be executed.
Variables specifed in the option "Custom environment variables" should be passed to rake tasks as well, as similar options work this way in other products known for developers and used for hosting Ruby apps like Heroku and DigitalOcean.