Release not run with Config.Reader Failure

I started with elixir/Phoenix and want to deploy a first demo (via docker) …

after mix release on my local machine there is error:

ERROR! Config provider Config.Reader failed with:
** (CompileError) _build/prod/rel/proberparameterfileassembler/releases/0.1.0/runtime.exs:1: module Mix.Config is not loaded and could not be found

i updated some days ago to Phoenix 1.6. . Before i never used the release feature. For dev it runs fine – really strange for me. I guess there was a change on configuration. How do i fix this ? The runtime.exs is a copy of dev.exs.

You must use import Config in runtime.exs (and preferably in other config files too), not use Mix.Config.