* `dev.exs`: default additional configuration for `MIX_ENV=dev`
* `prod.exs`: default additional configuration for `MIX_ENV=prod`
* `dev.exs`: default additional configuration for `MIX_ENV=dev`
* `prod.exs`: default additional configuration for `MIX_ENV=prod`
* `dev.secret.exs`: custom additional configuration for `MIX_ENV=dev`
* `prod.secret.exs`: custom additional configuration for `MIX_ENV=prod`
* `dev.secret.exs`: custom additional configuration for `MIX_ENV=dev`
* `prod.secret.exs`: custom additional configuration for `MIX_ENV=prod`