RHL-005-fix(env): update MongoDB connection string and clarify root auth requirements

.env.docker.example

@@ -4,8 +4,9 @@
 # NOTE: Never commit real secrets. Use placeholders in this example file.
 
 # MongoDB connection string inside the docker-compose network.
-# "mongo" should match the MongoDB service name in docker-compose.yml.
-MONGODB_URI=mongodb://mongo:27017/rhl_lieferscheine
+# "db" should match the MongoDB service name in docker-compose.yml.
+# If root auth is enabled (MONGO_INITDB_ROOT_USERNAME/PASSWORD), include credentials.
+MONGODB_URI=mongodb://root:replace-me@db:27017/rhl-lieferscheine?authSource=admin
 
 # Secret used to sign session JWT cookies.
 # Requirements: strong + random, at least 32 characters, never committed.