Created by: wanglei828
Base image, nvidia/cuda:9.0-cudnn7-runtime-ubuntu16.04, preinstalled higher version of libnccl, which will cause "apt-get install -y libnccl2=2.1.2-1+cuda9.0 libnccl-dev=2.1.2-1+cuda9.0" error. Add "--allow-downgrades" to fix this.