Fly.io doesn’t support private images, unfortunately, except for their own registry.fly.io.
https://community.fly.io/t/feature-request-images-from-private-docker-registries/842/2
The skopeo utility from the previous thread can copy from one to other, though. (I’ve used a variant of that several times myself, with Guix-built images.)
This is lighterweight alternative to using an entire Docker setup just to do the initial pull—which some other users were (understandably) dissatisfied with…