Remove MountFlags in systemd unit to allow shared mount propagation

Signed-off-by: Ilya Dmitrichenko <errordeveloper@gmail.com>

Author: errordeveloper

contrib/init/systemd/docker.service

@@ -11,7 +11,6 @@ Type=notify
 # for containers run by docker
 ExecStart=/usr/bin/dockerd -H fd://
 ExecReload=/bin/kill -s HUP $MAINPID
-MountFlags=slave
 LimitNOFILE=1048576
 LimitNPROC=1048576
 LimitCORE=infinity