Add APP_ENV env variable for possible override

Author: tcoch

compose.override.yaml

@@ -15,6 +15,7 @@ services:
       MERCURE_EXTRA_DIRECTIVES: demo
       # See https://xdebug.org/docs/all_settings#mode
       XDEBUG_MODE: "${XDEBUG_MODE:-off}"
+      APP_ENV: "${APP_ENV:-dev}"
     extra_hosts:
       # Ensure that host.docker.internal is correctly defined on Linux
       - host.docker.internal:host-gateway