I know lots of people have made stripped down versions of FreeBSD, e.g. nanobsd. Beyond that, I'm not aware of anything specific... but I probably wouldn't be

I know lots of people have made stripped down versions of FreeBSD, e.g. nanobsd. Beyond that, I'm not aware of anything specific... but I probably wouldn't be since I don't pay much attention to the container space. Try asking on one of the mailing lists maybe?