Detect Containerfiles as Dockerfiles

[ahills]

The podman & buildah tools support files named Containerfile in addition to Dockerfile, and the syntax is the same:

A Containerfile uses the same syntax as a Dockerfile internally.

https://docs.podman.io/en/latest/markdown/podman-build.1.html

According to the maintainers, the intent is to keep it the same:

The content, commands, and any features that are usable in a Dockerfile should also be usable in a Containerfile too. That's the design goal.

containers/buildah#3170

This patch simply extends the filename globs to include the alternative filenames & extensions.

[tancnle]

Thank you for your contribution @ahills ❤️ LGTM 🚀