poudriere-image generate wrong / owner with overlaydir option

[ocochard]

Prerequisites

• Have you checked for an existing issue describing your problem?
• Are you running the latest version?
• Is your ports tree recent?
• Is your FreeBSD Host on a supported release?

Describe the bug

Generating a poudriere-image with option `-c overlaydir`, when this directory belongs to a local system user, broke permission on the generated filesystem.

How to reproduce

Steps to reproduce the behavior:

with a local simple user:

mkdir -p /tmp/overlay/root
mkdir -p /tmp/overlay/etc
touch /tmp/overlay/etc/dummy1.txt
touch /tmp/overlay/root/dummy2.txt
sudo poudriere image ... -c /tmp/overlay

The generated image will have all existing directories belonging to an unknown user (because host local user id doesn’t exist on generated image), in this example its /, /etc and /root.
These bug is due to the cp -fRpr used.

Expected behavior

We could not preserve original user owner when copying the overlay directory: There is no local user created by default on the targeted disk image, and an already existing directory should not have this owner overwrited.

[ocochard]

A potential replacement of the cp could be something like (cd "${EXTRADIR}" && find . -print0 | cpio -pdm0 --no-preserve-owner ${WRKDIR}/world/)