Enable IPv6 on the telepresence docker network

[thallgren]

An apache running in a docker based intercept handler might fail with a DNS lookup failure when it dials other services in the cluster even though the DNS is resolved and returned. The reason appears to be that IPv6 isn't enabled, because when adding a network where it is, that resolves the problem.

The solution is to add IPv6 to the telepresence network, but doing that isn't straightforward because it requires that IPv6 has been enabled in /etc/docker/daemon.json, or that the network is created with an explicitly added IPv6 subnet and gateway. Let's say we create the telepresence network using --ipv6 . If it fails, I can see the following options:

1. inform the user that ipv6 must be added to /etc/docker/daemon.json
2. make another attempt without --ipv6, and log a warning that some DNS functionality might not be available.
3. make another attempt, but add an explicit subnet/gateway using --subnet and --gateway flag.

Number 3 is probably the most user-friendly, but it also requires Telepresence to select a random subnet, which defeats the purpose of defining this in /etc/docker/daemon.json.