Show how to utilize the provided .env file in the docker-compose file (…

…#50712)

### What?
I added a small change to the `with-docker-compose` example to show how to make the provided `.env` file usable.

### Why?
I struggled to understand why I could not get environment variables to work as intended and thought this small change might help someone else new to docker in the future.

### How?
Added a few comments to the `docker-compose.dev.yml` file, along with showing how to utilize a `.env` file with docker-compose



Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

examples/with-docker-compose/docker-compose.dev.yml

@@ -6,9 +6,15 @@ services:
     build:
       context: ./next-app
       dockerfile: dev.Dockerfile
+
+    # Set environment variables directly in the docker-compose file
     environment:
       ENV_VARIABLE: ${ENV_VARIABLE}
       NEXT_PUBLIC_ENV_VARIABLE: ${NEXT_PUBLIC_ENV_VARIABLE}
+
+    # Set envrionment variables based on the .env file
+    env_file:
+      - .env
     volumes:
       - ./next-app/src:/app/src
       - ./next-app/public:/app/public