Comment 3 for bug 1469204

not only... the variables that are in /etc/default/systemd are not being used by the docker.system file

I had to add the EnvironmentFile key (after having seen this thread/bug report)
 and also the $DOCKER_OPTS in the ExecStart value [originally it is NOT there]

[Unit]
Description=Docker Application Container Engine
Documentation=https://docs.docker.com
After=network.target docker.socket
Requires=docker.socket

[Service]
Type=notify
EnvironmentFile=-/etc/default/docker
ExecStart=/usr/bin/docker daemon $DOCKER_OPTS -H fd://
MountFlags=slave
LimitNOFILE=1048576
LimitNPROC=1048576
LimitCORE=infinity

[Install]
WantedBy=multi-user.target