Fix: Use variables instead of placeholders in docker-compose files

- Replace hardcoded placeholders with Docker Compose variables
- docker-compose.yml: Use ${DUCKDNS_SUBDOMAINS}, ${DUCKDNS_TOKEN}, ${DOMAIN}, ${SERVER_IP}
- traefik.yml: Use ${DEFAULT_EMAIL} for ACME email
- users_database.yml: Use ${AUTHELIA_ADMIN_USER}, ${AUTHELIA_ADMIN_EMAIL}, ${AUTHELIA_ADMIN_PASSWORD_HASH}
- sablier/docker-compose.yml: Fix to use ${DOMAIN} instead of {{DUCKDNS_DOMAIN}}

This aligns with the correct strategy:
- Docker-compose files use variables (${VAR})
- .env files use placeholders or actual values

docker-compose/core/traefik/traefik.yml

@@ -27,7 +27,7 @@ entryPoints:
 certificatesResolvers:
   letsencrypt:
     acme:
-      email: admin@example.com  # Your email for Let's Encrypt notifications
+      email: ${DEFAULT_EMAIL}  # Your email for Let's Encrypt notifications
       caServer: https://acme-v02.api.letsencrypt.org/directory  # Use staging for testing
       storage: /letsencrypt/acme.json
       # DNS challenge - For wildcard certificates (*.yourdomain.duckdns.org)