What is FreeBSD Jails?
The FreeBSD jail mechanism is an implementation of operating system-level virtualization that allows system administrators to partition a FreeBSD-based computer system into several independent mini-systems called jails.
FreeBSD Jails Screenshots
FreeBSD Jails information
Comments and Reviews
Said about FreeBSD Jails as an alternative
Great tools and documentation, they have been around for much longer than docker and are a more mature solution.