Individual Overlays for release builds

I am learning about releases. I see the :overlays option but maybe I am not understanding this. I tried something like this in mix.exs:

  defp releases do
      myapp: [
        overlays: ["myfile"]

but this raises error: ** (File.CopyError) could not copy recursively

Using dirs is ok, e.g. overlays: ["somedir/"], but how do I add individual files? I don’t wish them to be in sub-dirs. Also is it possible to use regex to copy all files matching a pattern?

Thank you!