"I would be totally on board with using FreeBSD as my primary server OS .. if it had a container engine."

FreeBSD is the original container engine - although, for the sake of completeness, we should count Solaris "zones" and call it a tie ...

I think there is a cultural and temporal connotation of Docker - and the use-cases that it typically enabled - that makes it seem very different than 'jail' but make no mistake: jail did all of those things and more, many years earlier.

The VPS, as we currently think of it, was built first[1] on top of FreeBSD and jail.

[1] JohnCompanies announced beta availability of "server instances" on 303 and cDc mailing lists in mid 2001. Someone else coined the term "VPS" a bit later ...