Since Mix is a build task, not a deploy task, the concept of a Mix environment doesn't exist in a release. You would need to set the release configuration based upon the Mix (or other) environment at build time. Maybe something like this is
def seeder(:dev), do: Seeder.Dev
def seeder(:prod), do: Seeder.Prod
Or better still refactor out to
prod.exs. Then in your pre-start scripts you would do:
seeder = Applcation.get_env(:my_app, :seed_module)
The point being "Mix.env()` is only a build time thing, not a release time thing.