cnest guide to nest containers and upgraded wrapper to podman/buildah

[castedo]

I've completed an upgrade and improved documentation for cnest, the podman/buildah wrapper that I've been dogfooding for the past two-ish years. It's similar to fedora toolbox and distrobox. I've written cnest.readthedocs.org to not just be about the cnest package, but also to be a guide about "nest containers" in general.

I hope some folks might benefit from either the docs, a trick or two from the code, or installing the cnest package.

Having implemented cnest, I have two nagging questions that have been bugging me:

• Is there a better choice than `sleep +Inf` for a "null" background container run cmd? #12934
• Any projects aiming for permissions control at a higher level than containers.conf, more like flatpak permissions? #12935

if people have insights or answers, it's much appreciated. And any feedback on the cnest package and/or documentation appreciated too.