feat: add IPv6 listeners

conf.d/default.conf.template

@@ -1,5 +1,6 @@
 server {
     listen 80 default_server;
+    listen [::]:80 default_server;
     server_name _;
 
     return 404;
@@ -7,6 +8,7 @@ server {
 
 server {
     listen 443 ssl default_server;
+    listen [::]:443 ssl default_server;
     include snippets/https.conf;
 
     server_name _;