Setting cookie with `RELEASE_COOKIE` env var, or with mix release configuration? Which is simpler?

The docs suggest to set a static cookie with a mix release configuration: Clustering Your Application · Fly Docs

I don’t understand why that is suggested, as it makes things quite difficult, especially if multiple release configs need to be created for different environments (each with their own cookie). In that case you also need to change your Dockerfile to accommodate for the RELEASE_NAME (the mix release command needs the name as input, and the release scripts in bin are also dependent on the RELEASE_NAME).

To me it seems way simpler to set the RELEASE_COOKIE environment variable, and be done with it. It takes precedence over the generated releases/COOKIE file. It’s documented here, when explaining the :cookie option. And in most cases it’s simple to specify a regular environment variable at runtime (or a secret, in my specific case).

Am I missing something, or is the suggestion on the documentation needlessly complicated?