RFC: Support other container runtimes

[aleksrutins]

I specifically have in mind Podman.

This could probably just be done as a fallback - if Docker's found, use it, else try Podman, etc.

Also, Podman has some interesting features that might be useful for Nixpacks - for instance, Buildah, which is a tool for building containers imperatively.

[coffee-cup]

This is a good idea. There is nothing in Nixpacks that should be Docker specific and supporting multiple container runtimes would ensure that. A first step would be to abstract the generation of the Dockerfile into a trait that could be implemented by a DockerGenerator or a PodmanGenerator.

[ghost]

Migrated to #121