You may be building runc / docker image puling from a private repository?
If so, Fly’s remote builders don’t yet support authenticating to those.
You can try building the runc image locally, like so:
flyctl deploy <other-args> --local-only
The default was --local-only before, but with one of the recent flyctl updates, they’ve switched it to --remote-only: Recent update prevents deploy from local docker image? - #2 by kurt