Caddy would have the bridge proxy network and the port 443 exposed.

version: "3.7"

networks:
  proxy-network:
    external: true
# needs to be created manually bevor running (docker create network proxy-network)
services:
  caddy:
    image: caddy
    container_name: caddy
    restart: unless-stopped
    ports:
      - 80:80
      - 443:443
    volumes:
      - ./data:/data
      - ./config:/config
      - ./Caddyfile:/etc/caddy/Caddyfile:ro
    networks:
      - proxy-network

Other services:

version: "3.7"

networks:
  proxy-network:
    external: true

services:
  app:
    image: app
    container_name: app
    restart: unless-stopped
    volumes:
      - ./app-data:/data
    networks:
      - proxy-network

Caddy can now talk to the app with the apps container_name.

Caddyfile:

homepage.domain.de {
    reverse_proxy app:80
}

So the reverse proxy network is an extra network only for containers that need to be exposed.