fix 1789: ensure that nginx resolves ipv4 addresses

core/nginx/conf/nginx.conf

@@ -18,7 +18,7 @@ http {
     keepalive_timeout  65;
     server_tokens off;
     absolute_redirect off;
-    resolver {{ RESOLVER }} valid=30s;
+    resolver {{ RESOLVER }} ipv6=off valid=30s;
 
     {% if REAL_IP_HEADER %}
     real_ip_header {{ REAL_IP_HEADER }};
@@ -233,7 +233,7 @@ mail {
     server_name {{ HOSTNAMES.split(",")[0] }};
     auth_http http://127.0.0.1:8000/auth/email;
     proxy_pass_error_message on;
-    resolver {{ RESOLVER }} valid=30s;
+    resolver {{ RESOLVER }} ipv6=off valid=30s;
 
     {% if TLS and not TLS_ERROR %}
     include /etc/nginx/tls.conf;