Dockerfile: use 'ENTRYPOINT' instead of 'CMD'

use entrypoint, so people can specify flags to etcd
without providing the binary.

Signed-off-by: Secret <haichuang221@163.com>

Dockerfile-release

@@ -6,5 +6,5 @@ RUN mkdir -p /var/etcd/
 
 EXPOSE 2379 2380
 
-# Define default command.
-CMD ["/usr/local/bin/etcd"]
+# Define default entrypoint.
+ENTRYPOINT ["/usr/local/bin/etcd"]