CentOS 7的配置文件位置略不同： 具体参见
https://www.daocloud.io/mirror#accelerator-doc中set_mirror脚本。
vi /usr/lib/systemd/system/docker.service
[Unit]
Description=Docker Application Container Engine
Documentation=http://docs.docker.com
After=network.target docker.socket
Requires=docker.socket
[Service]
Type=notify
# the default is not to use systemd for cgroups because the delegate issues still
# exists and systemd currently does not support the cgroup feature set required
# for containers run by docker
EnvironmentFile=/etc/sysconfig/docker
ExecStart=/usr/bin/docker daemon -H fd:// -H tcp://0.0.0.0:2375
MountFlags=slave
LimitNOFILE=1048576
LimitNPROC=1048576
LimitCORE=infinity
TimeoutStartSec=0
# set delegate yes so that systemd does not reset the cgroups of docker containers
Delegate=yes
[Install]
WantedBy=multi-user.target
生效服务：systemctl daemon-reload
service docker restart