错误原因：
/etc/docker/daemon.json文件有问题
情况1.里面有非法字符，删除多余的空格
情况2.daemon.json为空文件
错误现象：
[root@xdoctorx-7008 ~]# systemctl status docker
● docker.service - Docker Application Container Engine
Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled; vendor preset: disabled)
Active: Failed (Result: start-limit) since Fri 2020-04-17 21:17:58 CST; 1min 0s ago
Docs: https://docs.docker.com
Process: 7243 ExecStart=/usr/bin/dockerd -H unix:// (code=exited,status=1/FAILURE)
Main PID: 7243 (code=exited,status=1/FAILURE)
Apr 17 21:17:56 master systemd[1]: docker.service: main process exited,cod...RE
Apr 17 21:17:56 master systemd[1]: Failed to start Docker Application Conta...e.
Apr 17 21:17:56 master systemd[1]: Unit docker.service entered Failed state.
Apr 17 21:17:56 master systemd[1]: docker.service Failed.
Apr 17 21:17:58 master systemd[1]: docker.service holdoff time over,schedu...t.
Apr 17 21:17:58 master systemd[1]: Stopped Docker Application Container Engine.
Apr 17 21:17:58 master systemd[1]: start request repeated too quickly for d...ce
Apr 17 21:17:58 master systemd[1]: Failed to start Docker Application Conta...e.
Apr 17 21:17:58 master systemd[1]: Unit docker.service entered Failed state.
Apr 17 21:17:58 master systemd[1]: docker.service Failed.
Hint: Some lines were ellipsized,use -l to show in full.
[root@xdoctorx-7008 ~]# journalctl -xe
-- Subject: Unit docker.service has Failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit docker.service has Failed.
--
-- The result is Failed.
Apr 17 21:17:56 master systemd[1]: Unit docker.service entered Failed state.
Apr 17 21:17:56 master systemd[1]: docker.service Failed.
Apr 17 21:17:58 master systemd[1]: docker.service holdoff time over,scheduling
Apr 17 21:17:58 master systemd[1]: Stopped Docker Application Container Engine.
-- Subject: Unit docker.service has finished shutting down
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit docker.service has finished shutting down.
Apr 17 21:17:58 master systemd[1]: start request repeated too quickly for docker
Apr 17 21:17:58 master systemd[1]: Failed to start Docker Application Container
-- Subject: Unit docker.service has Failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit docker.service has Failed.
--
-- The result is Failed.
Apr 17 21:17:58 master systemd[1]: Unit docker.service entered Failed state.
Apr 17 21:17:58 master systemd[1]: docker.service Failed.
Apr 17 21:20:01 master systemd[1]: Started Session 38 of user root.
-- Subject: Unit session-38.scope has finished start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit session-38.scope has finished starting up.
--
-- The start-up result is done.
Apr 17 21:20:01 master CROND[7255]: (root) CMD (/usr/lib64/sa/sa1 1 1)
