Rootfs image from alternative sources

[Jip-Hop]

Currently the rootfs image is downloaded from linuxcontainers. Support additional rootfs image sources (at least when creating a jail from a template config file).

Currently the image is specified in the config template like so:

distro=debian
release=bookworm

Alternative sources could be from a URL or local filesystem, starting with file:// (could be dir or tar) or http(s)://. E.g.

image=https://mirror.serverion.com/fedora/releases/39/Container/x86_64/images/Fedora-Container-Base-39-1.5.x86_64.tar.xz

Perhaps machinectl could be used to download and extract the rootfs.