Build a custom dhparam file once for nginx and include it as default

plugins/nginx-vhosts/install

@@ -26,6 +26,12 @@ esac
 
 chmod 0440 /etc/sudoers.d/dokku-nginx
 
+# if dhparam.pem has not been created, create it the first time
+if [[ ! -f /etc/nginx/dhparam.pem ]]; then
+  openssl dhparam -out /etc/nginx/dhparam.pem 2048
+  chown root:root /etc/nginx/dhparam.pem
+fi
+
 # if dokku.conf has not been created, create it
 if [[ ! -f /etc/nginx/conf.d/dokku.conf ]]; then
   mkdir -p /etc/nginx/conf.d
@@ -40,6 +46,7 @@ ssl_session_cache shared:SSL:20m;
 ssl_session_timeout 1d;
 ssl_session_tickets on;
 
+ssl_dhparam /etc/nginx/dhparam.pem;
 ssl_ciphers ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:RSA+AESGCM:RSA+AES:!aNULL:!MD5:!DSS;
 
 EOF