docs: add tips to pass local environment variables to containers

docs/production.md

@@ -85,3 +85,18 @@ docker compose -f compose.yaml -f compose.prod.yaml up -d --wait
 If you want to deploy your app on a cluster of machines, you can use [Docker Swarm](https://docs.docker.com/engine/swarm/stack-deploy/),
 which is compatible with the provided Compose files.
 To deploy on Kubernetes, take a look at [the Helm chart provided with API Platform](https://api-platform.com/docs/deployment/kubernetes/), which can be easily adapted for use with Symfony Docker.
+
+## Passing local environment variables to containers
+
+By default, `.env.local` and `.env.*.local` files are excluded from production images.
+If you want to pass them to your containers, you can use the [`env_file` attribute](https://docs.docker.com/compose/environment-variables/set-environment-variables/#use-the-env_file-attribute):
+
+```yaml
+# compose.prod.yml
+
+services:
+    php:
+        env_file:
+            - .env.prod.local
+        # ...
+```