ref:
https://docs.docker.com/engine/installation/linux/ubuntulinux/#/install-the-latest-version
ps: if you want to use proxy in bash, type
export http_proxy="http://username:password@IP:Port"
ref: http://blog.csdn.net/zxncvb/article/details/42774987
加速器：
curl -sSL https://get.daocloud.io/daotools/set_mirror.sh | sh -s http://2a04ff3e.m.daocloud.io
sudo systemctl restart docker.service