/usr/local/jails - contains images
|- $jail_name
|- root
|- fstab
|- ...
/usr/local/share/jail
|- releases
|- 12.1-RELEASE - extracted base.txz system
cd into jailport directory, run
make install
it will produce working jail. You can change NAME and IP of jail, defining like this:
make NAME=jailname IP="lo1|10.0.0.1" install
Please inspect sourcecode before use.