tl;dr: following the steps of the Phoenix guide (on my computer) leads to a => ERROR [internal] load metadata for docker.io/hexpm/elixir:1.13.4-erlang-25.0.4-debian-bullseye-20210902-slim on fly launch build
Do you know what version of Phoenix you’re running? Phoenix 1.6.3 or later apps actually use the built in Phoenix Docker generator now (which was the result of the post about deploys being brittle). It works really well!
What error are you getting from the actual generated image? If it’s generating an image that doesn’t build, I think it’s worth a GitHub issue.
This is on Phoenix 1.6.11 and fly launch indeed seems to be generating the same Dockerfile as using mix phx.gen.release --docker. That Dockerfile is broken: as the code snippet above shows it interpolates an image tag that doesn’t exist on Dockerhub. Presumably because the Debian version is quite old.
What error are you getting from the actual generated image?
The image doesn’t build; see the first line of the OP. Or I’m misunderstanding your question.