I'm not sure I understand what dotenv does (or why you would need it to do it).
$ SECRET_TOKEN=abcdef SOME_OTHER_VAR=hello rails s
or pollute your .profile with a bunch of app-specific variables.
You don't need it to do it; it just makes it easier.
secret_token = File.read('secret_token_dont_check_in')
I fail to see how loading the file into the environment and then into the variable is anything but worse.