There are container environments such as GitHub codespaces that have
partial IPv6 support - routing support is enabled at the kernel level,
but lacking IPv6 filter support in the iptables module.
In the specific example of the codespaces environment, this also has
pre-existing legacy iptables rules in the IPv4 tables, as such the
nascent firewall mode detection will always pick iptables.
We would previously fault trying to install rules to the filter table,
this catches that condition earlier, and disables IPv6 support under
these conditions.
Updates #5621
Updates #11344
Updates #11354
Signed-off-by: James Tucker <james@tailscale.com>
055117ad45