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/authelia/config/users_database.yml

@@ -3,10 +3,10 @@
 ###############################################################
 
 users:
-  admin:
-    displayname: "admin"
-    password: "generate-with-openssl-rand-hex-64"
-    email: admin@example.com
+  ${AUTHELIA_ADMIN_USER}:
+    displayname: "${AUTHELIA_ADMIN_USER}"
+    password: "${AUTHELIA_ADMIN_PASSWORD_HASH}"
+    email: ${AUTHELIA_ADMIN_EMAIL}
     groups:
       - admins
       - users