feat(netlink): detect IPv6 using query to address

- If a default IPv6 route is found, query the ip:port defined by `IPV6_CHECK_ADDRESS` to check for internet access

Dockerfile

@@ -159,6 +159,8 @@ ENV VPN_SERVICE_PROVIDER=pia \
     FIREWALL_INPUT_PORTS= \
     FIREWALL_OUTBOUND_SUBNETS= \
     FIREWALL_DEBUG=off \
+    # IPv6
+    IPV6_CHECK_ADDRESS=[2606:4700::6810:84e5]:443 \
     # Logging
     LOG_LEVEL=info \
     # Health