Example of running FoundryVTT behind Caddy.

Caddyfile

{$SITE_ADDRESS} {
  reverse_proxy foundry:30000

  tls {$LETSENCRYPT_EMAIL} {
  # Comment out next line to use production CA
  ca https://acme-staging-v02.api.letsencrypt.org/directory
  }
}

docker-compose.yml

version: "3.8"

services:
  caddy:
    image: caddy:2-alpine
    volumes:
      - type: bind
        source: ./Caddyfile
        target: /etc/caddy/Caddyfile
      - type: bind
        source: ./volumes/caddy_data
        target: /data
      - type: bind
        source: ./volumes/caddy_config
        target: /config
    environment:
      - LETSENCRYPT_EMAIL=your_email@example.com
      - SITE_ADDRESS=vtt.example.com
    ports:
      - target: "443"
        published: "443"
        protocol: tcp
      - target: "80"
        published: "80"
        protocol: tcp

  foundry:
    image: felddy/foundryvtt:release
    hostname: my_foundry_host
    init: true
    restart: "unless-stopped"
    volumes:
      - type: bind
        source: ./volumes/foundry_data
        target: /data
    environment:
      - CONTAINER_CACHE=/data/container_cache
      - CONTAINER_VERBOSE=true
      - FOUNDRY_PASSWORD=<password>
      - FOUNDRY_USERNAME=<username>
      - FOUNDRY_ADMIN_KEY=atropos
      - FOUNDRY_PROXY_SSL=true