Scaling a service breaks because of a 'port already in use' error

A team runs docker compose up --scale app=3 -d on a service with ports: "8080:8080" and gets an error that the port is already allocated. Explain and fix. Expected reasoning: a fixed host port mapping can only be bound by one container; with 3 replicas, only the first succeeds. Fix: remove the explicit port mapping (or use expose: "8080" for internal-only visibility) and place a reverse proxy/load balancer container in front, publishing only the load balancer's port to the host, letting it distribute requests across the three app replicas via the project's internal DNS/network.