Fix nextcloud mail Message-ID header

docker/nextcloud/nginx.conf

@@ -129,6 +129,10 @@ http {
             fastcgi_pass php-handler;
             fastcgi_intercept_errors on;
             fastcgi_request_buffering off;
+
+            # Horde, used by nextcloud mail app, needs $_SERVER['SERVER_NAME'] to generate Message-ID header
+            # See also: https://github.com/nextcloud/mail/issues/5764
+            fastcgi_param SERVER_NAME $host;
         }
 
         location ~ ^\/(?:updater|oc[ms]-provider)(?:$|\/) {