That’s bonkers. VMs just use 8.8.8.8 to resolve addresses, I’m guessing your local docker was using a different nameserver? That’s the only difference I can think of. It’s especially weird that Deno.resolveDns worked.
Incidentally, if you control your Mongo servers might consider using WireGuard peers to connect to them from a Fly app. It’s a much better way to connect to DBs since peers are private and encrypted.