Default to empty DOCKER var so you wont accidently push to the
production dockerhub. But allow to be overriden to you can push
to your registry. This allows you to re-use the CoreDNS makefiles for
your internal CI/CD.

Hacked Makefile with echos:

make DOCKER=bla -f Makefile.release docker-push
Pushing: 1.0.1
for arch in amd64 arm arm64 ppc64le s390x; do \
    echo docker push bla/coredns:coredns-$arch ;\
done
docker push bla/coredns:coredns-amd64
docker push bla/coredns:coredns-arm
docker push bla/coredns:coredns-arm64
docker push bla/coredns:coredns-ppc64le
docker push bla/coredns:coredns-s390x
echo manifest-tool push from-args --platforms linux/amd64,linux/arm,linux/arm64,linux/ppc64le,linux/s390x --template bla/coredns:coredns-ARCH --target bla/coredns:1.0.1
manifest-tool push from-args --platforms linux/amd64,linux/arm,linux/arm64,linux/ppc64le,linux/s390x --template bla/coredns:coredns-ARCH --target bla/coredns:1.0.1
echo manifest-tool push from-args --platforms linux/amd64,linux/arm,linux/arm64,linux/ppc64le,linux/s390x --template bla/coredns:coredns-ARCH --target bla/coredns:latest
manifest-tool push from-args --platforms linux/amd64,linux/arm,linux/arm64,linux/ppc64le,linux/s390x --template bla/coredns:coredns-ARCH --target bla/coredns:latest