[Service]
Environment="DOCKER_OPTS=--insecure-registry registry.service.consul:5000 --ipv6"

[Unit]
Requires=etcd2.service
After=etcd2.service

[Service]
Environment="DOCKER_OPTS=--insecure-registry registry.service.consul:5000 --ipv6 --cluster-store=etcd://127.0.0.1:2379"

Use etcd as Docker's --cluster-store
Use etcd as Docker's --cluster-store

This is required for Calico: https://docs.docker.com/swarm/networking/
