When it comes to DHCPv6, Alpine might not be your first choice. Proxmox VE is still accusing the udhcpc6 in BusyBox, a DHCP implementation used by Alpine is unfinished and broken since early 2016, and all interfaces marked as dhcp is automatically turned into manual mode. While I have not really dig into whether the udhcpc6 in Alpine actually works or not, changing the /etc/network/interfaces to again mark the interface as dhcp does not do its magic. Alternatively, I will need to install another DHCP client for the purpose.
From the Proxmox VE source code:
perl
sub setup_network {
# Network is debian compatible, but busybox' udhcpc6 is unfinished
my ($self, $conf) = @_;
# XXX: udhcpc6 in busybox is broken; once a working alpine release comes
# we can remove this bit.
#
# Filter out ipv6 dhcp and turn it into 'manual' so they see what's up.
#
# XXX: slaac works different from debian - busybox has no 'auto'
# configuration type - https://wiki.alpinelinux.org/wiki/Configure_Networking#IPv6_Stateless_Autoconfiguration
# the suggested configuration sadly does not take the interface up, but
# at least with the workaround the networking starts and if an ipv4 is
# configured slaac for ipv6 works (unless accept_ra = 0 in the node)A bit of search turns that there is another implementation
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