Backend podman unsupported

[kkaempf]

While the BACKEND variable docs say

# Backend to use for "luet build"
# Values "docker" or "podman"

actually setting BACKEND=podman results in

> make 
PACKAGES >live/boot live/syslinux live/systemd-boot system/cos<
/home/kkaempf/bin/luet build --pull --no-spinner --only-target-package --live-output --image-repository raccos/opensuse \
--values /abuild/projects/rancher/cOS/cOS-toolkit/values/opensuse.yaml \
--tree=/abuild/projects/rancher/cOS/cOS-toolkit/packages \
--backend podman \
--concurrency 1 \
--compression zstd \
--destination /abuild/projects/rancher/cOS/cOS-toolkit/build \
        live/boot live/syslinux live/systemd-boot system/cos
 invalid backend. Unsupported
make: *** [make/Makefile.build:41: build] Error 1