DS215+ When building 7.1 for alpine4k it actually builds packet for alpine

[acordtag]

using
git clone https://github.com/runfalk/synology-wireguard.git && cd synology-wireguard/ && sudo docker build -t synobuild .
and
mkdir $(pwd)/artifacts
sudo docker run --rm --privileged --env PACKAGE_ARCH=alpine4k --env DSM_VER=7.1 -v $(pwd)/artifacts:/result_spk synobuild

it works fine but the end result is alpline not alpine4k. Any idea how to fix it?