diff --git a/Dockerfile b/Dockerfile
index c4de7405592f0f76b86b7efa0d90ee97f7575e87..fbec88c7966d6ea93495519843d6cda63f622661 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -53,6 +53,10 @@ RUN localedef -i en_US -f UTF-8 en_US.UTF-8
 
 # FIXME: due to temporary ipykernel dependency issue, specify ipykernel jupyter
 # version util jupyter fixes this issue.
+
+# specify sphinx version as 1.5.6 and remove -U option for [pip install -U
+# sphinx-rtd-theme] since -U option will cause sphinx being updated to newest
+# version(1.7.1 for now), which causes building documentation failed.
 RUN pip install --upgrade pip && \
     pip install -U wheel && \
     pip install -U docopt PyYAML sphinx==1.5.6 && \