Systemd init script should support DOCKER_OPTS

See moby#9889 (specifically, [here](moby#9889 (comment)). The systemd daemon should support `DOCKER_OPTS` the same as upstart and sysv do.

This commit is copied from
moby@0e7fb27
The commit itself is not present anymore (maybe it's orphaned?).

Signed-off-by: Matthias Brandt <matthias.brandt@inoio.de>

contrib/init/systemd/docker.service

@@ -6,8 +6,9 @@ Requires=docker.socket
 
 [Service]
 Type=notify
-ExecStart=/usr/bin/docker daemon -H fd://
 MountFlags=slave
+EnvironmentFile=-/etc/default/docker
+ExecStart=/usr/bin/docker daemon $DOCKER_OPTS -H fd://
 LimitNOFILE=1048576
 LimitNPROC=1048576
 LimitCORE=infinity