As for why Ctrl+Z suspend doesn't work, perhaps
poudriere-3.3.6/src/poudriere-sh/helpers.c is trapping it.
However, it does trap SIGTSTP, but not SIGSTOP.
In other words, Ctrl+Z with SIGTSTP will not suspend, but SIGSTOP will.
I actually used the following method to suspend it.
# Find the jail
jls -v
(snip)
87 foojail-default-job-01
/usr/local/poudriere/data/.m/foojail-default/01
foojail-default-job-01 ACTIVE
5
127.0.0.1
::1
# suspend all processes.
pkill -STOP -j 87 -a
# restart the process
pkill -CONT -j 87 -a
# note: this will not suspend
pkill -TSTP -j 87 -a
