tl;dr

# CLI
git clone https://github.com/jhfoo/freebsd-containers.git
cd freebsd-containers
./bin/build-custom 

Key steps

# CLI: download latest base image
fetch https://download.freebsd.org/ftp/releases/amd64/14.3-RELEASE/base.txz

# Containerfile
FROM scratch
ADD base.txz /
ENTRYPOINT /bin/sh

# CLI: build with podman
sudo podman build -t localhost/freebsd-custom:latest .

Tested

1. Able to upload to registry (tested: forgejo).