Enable TLSv1.3 finally

With Alpine 3.9 https://pkgs.alpinelinux.org/package/v3.9/main/x86/openssl we got OpenSSL 1.1.1a. 
With https://github.com/docker-library/official-images/pull/5377 it was merged into the Nginx upstream image and thus Nginx was built with it.

data/conf/nginx/site.conf

@@ -14,7 +14,7 @@ server {
 
   ssl_certificate /etc/ssl/mail/cert.pem;
   ssl_certificate_key /etc/ssl/mail/key.pem;
-  ssl_protocols TLSv1.2;
+  ssl_protocols TLSv1.2 TLSv1.3;
   ssl_ciphers 'ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256';
   ssl_prefer_server_ciphers on;
   ssl_session_cache shared:SSL:50m;