Multiple HTTPS sites on Gigalixir using master_proxy

I made a post before 1 Gigalixir instance hosting multiple HTTPS sites - #5 by axelson
but I am still not understanding how can I support https. The multi_proxy config was a little bit difficult, but I think as long as your endpoints do not specify a port and they let multi_proxy define the port, it works. But if you have https, I do not see what options are needed (or maybe, it isn’t needed?)

config :master_proxy,
  http: [port: 80],
  #  http: [port: String.to_integer(System.get_env("PORT", "4000"))],  # <--- is this better?
  #  https: [:inet6, port: 443],   # <--- this causes an error.  How to specify a local cert?
  backends: [
    %{
      host: ~r{^(www\.)?myapp\.com$},
      phoenix_endpoint: MyAppWeb.Endpoint
    },
  ]

If https is uncommented, it gets this error:

(ArgumentError) could not start Cowboy2 adapter, missing option :key/:keyfile

but I do not understand how to fix this in dev.

Also I do not understand whether master_proxy should be listening on port 4000 (the PORT), or if it should be listening on port 80.

Thank you sorry I am confused