docker-compose: comment about DB_HOST

.env

@@ -9,6 +9,8 @@ INDEXER_DB_NAME=query_node_indexer
 DB_NAME=query_node_processor
 DB_USER=postgres
 DB_PASS=postgres
+# This value will not be used by query-node docker containers.
+# When running query-node with docker these services will always use the db service
 DB_HOST=localhost
 DB_PORT=5432
 DEBUG=index-builder:*