Getting started example throws error, docs.fedoraproject.org example works

[PackElend]

Hello,
on Raspberry Pi 4 running latest Raspi OS the following commands:
https://github.com/coreos/butane/blob/main/docs/getting-started.md

podman run --rm \
       -v config.bu:/config.bu:z quay.io/coreos/butane:release \
       --pretty --strict /config.bu > onfig.ign

throws
failed to read /config.bu: read /config.bu: is a directory

using

podman run --interactive --rm --security-opt label=disable \
       --volume ${PWD}:/pwd --workdir /pwd quay.io/coreos/butane:release \
       --pretty --strict config.bu > config.ign

from Producing an Ignition Config :: Fedora Docs works well