Simply:

• releases.exs - runtime configuration for release only
• #{Mix.env()}.exs - i.e. dev.exs, prod.exs, test.exs - those are compiletime environment-specific configuration files
• config.exs - compiletime generic configuration file
• releases.exs - runtime generic configuration file

Please look that machine which compiles code does not needs to run it. When calling System.get_env/1 it’s not executed in some kind of configuration queue, but immediately! This means that machine building code (regardless if it would run it or not) must have specified all environment variables.

Obviously it causes many troubles especially when same project was deployed to multiple machines when each of such machine wanted different environment configuration. It has been partially solved by releases.exs which now should be replaced by more generic runtime.exs.

I recommend to watch this video:

My opinion in a nutshell:

If you are on Elixir v1.11, use config/runtime.exs for all configuration by default. Make files like config/runtime_dev.exs as needed and use config_env/0 to check which one to import. (EDIT: Turns out config/runtime.exs doesn’t support imports.) Only move configuration to config/config.exs that you know that you need to set at compile time.

This should give you the easiest path forward as your configuration will behave the same way in dev, test, prod, and release by default.

The problem is knowing which files need copying. Unless imported files can be statically determined at buildtime the release won‘t be able to know which config files to include additionally.