poudriere-jail(8): a working example of values for -m and -U for pkgbase

From: Graham Perrin <grahamperrin_at_gmail.com>
Date: Sat, 27 Apr 2024 13:54:10 UTC
Please, can someone provide a truly usable combination of values to 
create a 15.0-CURRENT amd64 jail named 'main'?

Following the recent commits, I performed more than nine experiments. In 
each case the end result was "Jail main 15.0-CURRENT amd64 is ready to 
be used" for a jail that was _not_ ready.

Thank you.

Context:

% pkg -vv | grep -e url
     url             : 
"pkg+https://pkg.freebsd.org/FreeBSD:15:amd64/latest",
     url             : 
"pkg+https://pkg.freebsd.org/FreeBSD:15:amd64/base_latest",
     url             : 
"file:///usr/local/poudriere/data/packages/main-default",
% pkg iinfo poudriere
poudriere-devel-3.4.99.20240424
%