I think that has to do with it being more of a cultural change rather than a code change. I think as long as Podman/Buildah support both filenames by default,it would be the users choice about what to name the files. If they wanted to make their project less visually tied to Docker, they could use Containerfile, or if they wanted to keep things as-is and slowly move things over (or never move things), that's fine too. I think it'll be up to us to promote the use of "Containerfile" as a specification for using this format in a vendor-neutral way. The OCI folks in their IRC channel pointed me to
https://buildpacks.io/ as a CNCF project for creating a abstracted universal build standard for OCI images. I think it's a great idea, but I really couldn't see many organizations being able to quickly turn over their container infrastructure to use buildpacks as opposed to the Dockerfile-type format.