docker+machine executor: IPv6 support

Description

gitlab-runner doesn't allow to enable IPv6 support in the docker runners following suggestions of the docker documentation (link).

Proposal

Add in [[runners.docker]] options corresponding to the Docker documentation:

  ipv6= true
  fixed-cidr-v6= "2001:db8:1::/64"
  experimental= true
  ip6tables = true

which would be propagated to the Docker daemon's configuration on the provisioned machines.

The container running on such Docker daemon would have the IPv6 addresses assigned, be part of the dedicated IPv6 isolated subnet (specified by 'fixed-cidr-v6') and have routing to the Internet.