Some people might want to use Gitlab’s support for spinning up CI runners on DigitalOcean, and may want to test their application on IPv6… Well, Docker, by default, does not do IPv6: you have to add --ipv6=true to the daemon, and add some CIDR that Docker may hand out (e.g. fixed-cidr-v6='fc00::d0c:0:0:0:1/64').

If you’re okay with link local IPv6, you can use these options in your Gitlab Bastion:

    MachineOptions = [
      "digitalocean-image=coreos-stable", # The DigitalOcean system image to use by default
      "digitalocean-ssh-user=core", # The default SSH user
      "digitalocean-access-token=ACCESS_TOKEN", #     Access token from Step 7
      "digitalocean-region=ams3", # The data center to spawn runners in
      "digitalocean-size=c-1vcpu-2gb", # The size (and price category) of your spawned runners
      "digitalocean-private-networking", # Enable private networking on runners

The trick is to pass an extra = between engine-opt and the option itself, otherwise Docker Machine interprets 'engine-opt ipv6=true' as a standalone option, instead of 'engine-opt' as option and 'ipv6=true' as parameter.

If not, sad news, you have to wait for Better IPv6 support from the Docker Machine, because you’d have to pass your public prefix in the Gitlab config…