...
ExecStart=/usr/bin/dockerd -H fd:// -H tcp://0.0.0.0:2376 -H unix:///var/run/docker.sock\
  --tlsverify \
  --tlscacert=/etc/docker/certs/ca.pem \
  --tlskey=/etc/docker/certs/key.pem \
  --tlscert=/etc/docker/certs/cert.pem
...