diff --git a/tools/dockerfile/test/cxx_clang_7_x64/Dockerfile b/tools/dockerfile/test/cxx_clang_7_x64/Dockerfile
index 67b60db67cce4..2454777b55d57 100644
--- a/tools/dockerfile/test/cxx_clang_7_x64/Dockerfile
+++ b/tools/dockerfile/test/cxx_clang_7_x64/Dockerfile
@@ -62,7 +62,7 @@ RUN apt-get update && apt-get -y install libc++-dev clang && apt-get clean
 RUN apt-get update && apt-get install -y cmake && apt-get clean
 
 #=================
-# Install ccache
+# Install ccache (use slighly older release of ccache that works older compilers and cmake)
 
 # Install ccache from source since ccache 3.x packaged with most linux distributions
 # does not support Redis backend for caching.