I got the same error today, fresh Phoenix app following the Fly.io guide.
I looked up the default tag 1.13.4-erlang-25.0.4-debian-bullseye-20210902-slim
on Docker Hub and saw that it didn’t exist. Updating my Dockerfile
to point to 1.13.4-erlang-25.0.4-debian-bullseye-20220801-slim
instead allowed me to fly deploy
.