1. build.sh extrahieren
(sudo) docker run --rm --entrypoint /bin/bash blackvoidclub/synobuild73 -c "cat /source/WireGuard/build.sh" > /local/path/build.sh.fixed

2. change line 27:
from
export LIBMNL_VERSION=$(wget -q 'https://www.netfilter.org/projects/libmnl/files/?C=M;O=D' -O - | grep -oP 'a href="libmnl-\K[0-9.]*' | head -n 1 | sed 's/.\{1\}$//')
to
export LIBMNL_VERSION=$(wget -q 'https://www.netfilter.org/pub/libmnl/?C=M;O=D' -O - | grep -oP 'a href="libmnl-\K[0-9.]*' | head -n 1 | sed 's/.\{1\}$//')

3. create a dockerfile:
/bin/bash:
cat > /local/path/dockerfile.fixed << 'EOF'
FROM blackvoidclub/synobuild73
COPY /local/path/build.sh.fixed /source/WireGuard/build.sh
RUN chmod +x /source/WireGuard/build.sh
EOF

4. build new docker:
(sudo) docker build -f /local/path/dockerfile.fixed -t mycontainer .

5. run new fixed dockerimage:
docker run \
--rm \
--privileged \
--env PACKAGE_ARCH=<yourNASarch> \
--env DSM_VER=7.3 \
-v /volume1/docker/synowirespk73:/result_spk \
mycontainer:latest

build WireGuard-<yourNASarch>-1.0.20220627.spk successfully - May'26

Thanks to: https://www.blackvoid.club/wireguard-spk-for-your-synology-nas/