Write logs for production to webapp/var/log/prod.log instead of stderr.

webapp/config/packages/monolog.yaml

@@ -60,9 +60,8 @@ when@prod:
                 buffer_size: 50 # How many messages should be saved? Prevent memory leaks
             nested:
                 type: stream
-                path: php://stderr
+                path: "%kernel.logs_dir%/%kernel.environment%.log"
                 level: debug
-                formatter: monolog.formatter.json
             console:
                 type: console
                 process_psr_3_messages: false