diff --git a/docker-compose.prod.yml b/docker-compose.prod.yml
index 72ac16c..87df349 100644
--- a/docker-compose.prod.yml
+++ b/docker-compose.prod.yml
@@ -4,6 +4,9 @@
 # Base images use DaoCloud public mirror (大陆可访问): docker.io → docker.m.daocloud.io
 # See https://github.com/DaoCloud/public-image-mirror
 #
+# The ``api`` service is built from ``Dockerfile``: Debian apt → 阿里云, uv/PyPI → 清华,
+# PyTorch CPU wheels / pytorch.org index files → 南大镜像; see Dockerfile for details.
+#
 # Published API port defaults to 38080 on the host (override with API_PORT).
 #
 # GPU (NVIDIA) inference: uv.lock pins torch/torchvision from the PyTorch *CPU* index (see pyproject.toml).