How to add .env file or otherwise set environment variables in a Heroku app?

There is no need for an .env file on Heroku. In fact, such a file won't work very well since

• Heroku gets all of its files from your Git repository,
• has an ephemeral filesystem, meaning that changes to files like .env will be quickly lost, and
• the .env file won't be available on other dynos if you scale your app

As such, creating an .env file on Heroku isn't a good approach.

Instead, you can use its built-in support for environment variables, using heroku config:set <var> <value> or its web UI. Either way, you'll get a regular environment variable.