Makefile: Fix build-docker-release-master
Makefile: Fix build-docker-release-master

Since the Dockerfile files are now per arch, this adjusts to detect ARCH
and builds docker release from the Dockerfile.<ARCH> file.

coreos/etcd,mqliang/etcd,mkumatag/etcd,ajityagaty/etcd,mqliang/etcd,ajityagaty/etcd,rnaveiras/etcd,gyuho/etcd,etcd-io/etcd,rnaveiras/etcd,rnaveiras/etcd,ajityagaty/etcd,coreos/etcd,mkumatag/etcd,etcd-io/etcd,gyuho/etcd,mqliang/etcd,mkumatag/etcd,etcd-io/etcd