Loading an umbrella app's own runtime.exs in dev

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?

I borrowed the below from here. I have very little in the app specific configs. Just the things that clash, e.g. swoosh and sentry.

  defp releases do
    target = target()
    [
      localapp: [
        include_executables_for: executables(target),
        applications: [runtime_tools: :permanent, localapp: :permanent],
        steps: steps(target),
        rel_templates_path: "apps/localapp/rel"
      ],
      cloudapp: [
        include_executables_for: [:unix],
        applications: [runtime_tools: :permanent, cloudapp: :permanent],
        steps: steps(:unix),
        rel_templates_path: "apps/cloudapp/rel",
        config_providers: config_providers_for_apps([:cloudapp])
      ]
    ]
  end

  # Add the runtime.exs configuration for each provided app
  defp config_providers_for_apps(apps) do
    for app <- apps do
      {Config.Reader,
       path: {:system, "RELEASE_ROOT", "/apps/#{app}/config/runtime.exs"}, env: Mix.env()}
    end
  end

  # When assembling the release, we copy all the runtime.exs files defined in `config_providers` to it,
  # keeping the relative app path to avoid collisions.
  defp copy_configs(%Mix.Release{path: release_dir, config_providers: config_providers} = release) do
    for {_module, path: {_context, _root, config_file_path}, env: _} <- config_providers do
      config_dir = Path.join(release_dir, Path.dirname(config_file_path))

      # Clean the config directory to make sure we are only including the files defined in the config_providers
      File.rm_rf!(config_dir)
      File.mkdir_p!(config_dir)

      File.cp!(
        Path.relative(config_file_path),
        Path.join(config_dir, Path.basename(config_file_path))
      )
    end

    release
  end