Yum update & overwriting config files


I configured docker (conform to its documentation for systemd https://docs.docker.com/articles/systemd/).
However after update:

[root@db7 ~]# cat /etc/redhat-release 
CentOS Linux release 7.1.1503 (Core) 

yum update -y
---> Package docker-engine.x86_64 0:1.8.1-1.el7.centos will be updated
---> Package docker-engine.x86_64 0:1.8.2-1.el7.centos will be an update
docker-engine.x86_64 0:1.8.2-1.el7.centos

the custom content of my config files (i.e. /etc/sysconfig/docker, /usr/lib/systemd/system/docker.service) have been lost.

Is this correct or I did omit sth?

best regards,