问题：DOCKER_OPTS参数不生效
最近更新了docker版本Docker version 18.03.1-ce, build 9ee9f40，更新docker源的时候发现，修改/etc/default/docker后，使用docker info 查看相关配置，配置无更新，即DOCKER_OPTS参数无法生效。
原因
在新的版本中，/etc/default/docker默认不生效=。= 需要进行手动配置，使其生效。
官方说明：
## 1.12.0 (2016-07-28)
**IMPORTANT**: Docker 1.12 ships with an updated systemd unit file for rpm
based installs (which includes RHEL, Fedora, CentOS, and Oracle Linux 7). When
upgrading from an older version of docker, the upgrade process may not
automatically install the updated version of the unit file, or fail to start
the docker service if;
- the systemd unit file (`/usr/lib/systemd/system/docker.service`) contains local changes, or
- a systemd drop-in file is present, and contains `-H fd://` in the `ExecStart` directive
虽然说文档上说docker.service这个文件会在/usr/lib/systemd/system/下，但其真实路径是路径是/lib/systemd/system/。该路径是在默认的安装脚本中指定的systemd/docker.service lib/systemd/system/。
解决
1、在/lib/systemd/system/docker.service中添加EnvironmentFile=-/etc/default/docker，并修改ExecStart参数为ExecStart=/usr/bin/dockerd -H fd:// $DOCKER_OPTS
# 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/default/docker
ExecStart=/usr/bin/dockerd -H fd:// $DOCKER_OPTS
ExecReload=/bin/kill -s HUP $MAINPID
2、重启docker-daemon
systemctl daemon-reload
3、重启docker
service docker restart