Wish they'd enable IPv6 by default
https://docs.docker.com/articles/networking/#ipv6 has more of the details (and the discussion at https://github.com/docker/docker/pull/8947#discussion_r22534... is also useful)
Basically, we can't use existing router advertisements (as I understand it) because you also have to tell your current IPv6 router that the entire prefix you use for Docker needs to go to this one host as opposed to just the one IPv6 address that host would auto-assign itself via RA.
Since there's manual outside-Docker setup involved, we can't really automate this bit. If there's a nice clean way to do so, we're definitely open to a PR (I'd love to have something simpler myself)! :)
Will have to take a look, I guess there are lots of potential setups. If you have a /64 per host it should be ok anyway, if you have a /64 for the network it might not be.
LL addresses are used for stuff like router advertisement, neighbour discovery (IPv6 equivalent of ARP) etc. You can't use link-local addresses without extra gyrations in socket API (scope id) so they cannot be usefully passed to normal apps.
I think the issues are more around the fact you may need to change existing addresses, plus potentially split up the /64 (or whatever) your machine has among docker bridges, plus you need to make sure the /64 is no longer on the external interface.