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/sablier/docker-compose.yml

@@ -9,7 +9,7 @@ services:
       - /var/run/docker.sock:/var/run/docker.sock:ro
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.sablier.rule=Host(`sablier.{{DUCKDNS_DOMAIN}}`)"
+      - "traefik.http.routers.sablier.rule=Host(`sablier.${DOMAIN}`)"
       - "traefik.http.routers.sablier.entrypoints=websecure"
       - "traefik.http.routers.sablier.tls=true"
       - "traefik.http.routers.sablier.tls.certresolver=letsencrypt"