> And what about if my Linux Distro doesn't use system-md, like Slackware.
It's a choice of Slackware not use systemd infra. However on Linuxes you have more that one choice to run containers, your next option would be LXC/LDX https://docs.slackware.com/howtos:misc:lxc
> However both are pretty much mimicked concept from Solaris Zones
Your timeline is off.
FreeBSD and Linux were within a year of each other (not in-tree on linux, but Debian packaged VServer kernels and VServer + GRSec kernels; we used vserver+grsec debian packaged kernels at work in the mid through late '00s).
It's a choice of Slackware not use systemd infra. However on Linuxes you have more that one choice to run containers, your next option would be LXC/LDX https://docs.slackware.com/howtos:misc:lxc