> +is_available () {
> +    # Use the "type" shell builtin to see if the program is on PATH.
> +    type "$1"

Maybe >/dev/null ?  Then it would only complain if it needed to.

> +else
> +    echo "skipping pack verification because chroot or unshare is unavailable"

echo "warning: Skipping pack verification because chroot or unshare is unavailable" >&2

Otherwise LGTM!