add milvus build environment docker-entrypoint.sh

Former-commit-id: de3e29691876adfae48a6b55b5dc2e4d7ee99b3e

docker/build_env/ubuntu16.04/docker-entrypoint.sh

+#!/bin/bash
+
+set -e
+
+if [ "$1" = 'start' ]; then
+   tail -f /dev/null
+fi
+
+exec "$@"
+