Fixing IPv6 neighbor discovery in Proxmox Containers

Disable multicast snooping with:

echo -n 0 > /sys/class/net/<BRIDGE>/bridge/multicast_snooping

To make the change persistent, use bridge-mcsnoop 0 in /etc/network/interfaces like so:

auto vmbr0
iface vmbr0 inet manual
    bridge-ports bond1
    bridge-stp on
    bridge-fd 15
    bridge-vlan-aware yes
    bridge-vids 2-4094
    bridge-mcsnoop 0