LOTS of bad info on this. Its really pretty simple. When configuring the system (assuming your isp has SLAAC setup!) simply choose 'setup' eth0 with NO ipv4 address. things should 'just work' after that. oh and be sure to put in an ipv6 dns server! Also be sure to wait a bit since SLAAC can take a bit if you are using a macvlan passthrough for a network via vm.
auto eth0
iface eth0 inet manual
up ip link set $IFACE up
down ip link set $IFACE downYou just things to 'manual' and not setup any ip address. LOTS of old things don't work, and 'slaac' now works out of the box for alpine linux 3.19.