Up to now I've only used dotenv in a Rails context. It can just as easily be used in a plain old Ruby project.
Install the non-Rails version of the gem.
$geminstalldotenv
Then add the following lines wherever you want dotenv included and loaded. In my case, I want it pulled in as part of my RSpec setup in spec_helper.rb.
require'dotenv'Dotenv.load
Your environment variables declared in .env are now accessible via fetches against the ENV object.