Evaluate the new FreeBSD docker containers to replace the qemu solution

Since gitlab-runner 16.9 (just released) + podman 5.0 (expected in the next couple of weeks), FreeBSD docker containers are supported (see https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/4551; the MR description has instructions as well)

This could make all the qemu/vmctl stuff from !114 (merged) no longer necessary.