Document that IPv6 might break when docker is configured for IPv6 and how to fix it

[Syndace]

Relevant issue in the docker repos: https://github.com/docker/for-linux/issues/844

Short summary of the issue: enabling docker IPv6 support can drop the default route if the route was obtained via router advertisement, rendering IPv6 on the host unusable. This is due to the fact that docker enables IPv6 forwarding, which in turn drops routes obtained via router advertisement, unless net.ipv6.conf.<iface>.accept_ra=2 is set during boot.

This is not the fault of mailcow, but could affect mailcow users (like me) and send them on an hourlong hunt (again, like me :D), so I'd appreciate that being added to the documentation, possibly in the "General Troubleshooting" section.

A post explaining a different solution: https://strugglers.net/~andy/blog/2011/09/04/linux-ipv6-router-advertisements-and-forwarding/

[MAGICCC]

Feel free to create a PR :)