I’m not sure how exactly certificates are served by gigalixir. It could be one certificate for all domains, but it could be one certificate per domain and it serves only the matching one.
But all that doesn’t matter at all if your dns provider doesn’t allow root level CNAMES. Because you’re likely not even serving gigalixir resources under https://myapp.com. Meaning neither gigalixir nor phoenix can do anything, as things go the wrong way on the dns level already. I’m not sure if you can get an IP for gigalixir. If not you need to either find something similar to CNAME (e.g. ALIAS), which is supported by GoDaddy or move to a different DNS provider.
