Dockerfile-release: add '/var/lib/etcd/'

We have '/var/etcd/' in Dockerfile for historical reason.
Most cases, user store data in '/var/lib/etcd/'.

Dockerfile-release

@@ -3,6 +3,7 @@ FROM alpine:latest
 ADD etcd /usr/local/bin/
 ADD etcdctl /usr/local/bin/
 RUN mkdir -p /var/etcd/
+RUN mkdir -p /var/lib/etcd/
 
 EXPOSE 2379 2380