Given an umbrella application with applications that define their own runtime.exs files in apps/appname/config/runtime.exs

Context:

When I created my own umbrella project, (e.g. mix phx.new --umbrella) , the directory structure ended up something like the following.

my_app_umbrella
├── README.md
├── apps
│   ├── my_app
│   └── my_app_web
├── config
│   ├── config.exs
│   ├── dev.exs
│   ├── prod.exs
│   ├── runtime.exs
│   └── test.exs
└── mix.exs

And trying any sort of import_config inside config/runtime.exs looks to be forbidden

import_config "../apps/my_app/config/runtime.exs"

or if I have config/my_app/runtime.exs closer to the config files in the umbrella app itself:

import_config "my_app/config/runtime.exs"

% iex -S mix                          
** (RuntimeError) import_config/1 is not enabled for this configuration file. Some configuration files do not allow importing other files as they are often copied to external systems
    (elixir 1.16.2) lib/config.ex:312: Config.import_config!/3
    /Users/ben/my_app_umbrella/config/runtime.exs:112: (file)
    (stdlib 5.2) erl_eval.erl:750: :erl_eval.do_apply/7
    (stdlib 5.2) erl_eval.erl:136: :erl_eval.exprs/6
    (elixir 1.16.2) lib/code.ex:572: Code.validated_eval_string/3

I had assumed that for umbrella applications, the way it does releases, to place all config in this runtime.exs.

The app specific config files are added to the releases using Config Providers.

Can you provide some additional context about how your Config Providers work?