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