diff --git a/Dockerfile.tmpl b/Dockerfile.tmpl
index a7831a81..449dfd2c 100644
--- a/Dockerfile.tmpl
+++ b/Dockerfile.tmpl
@@ -83,6 +83,7 @@ RUN curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key --
 # Use a fixed apt-get repo to stop intermittent failures due to flaky httpredir connections,
 # as described by Lionel Chan at http://stackoverflow.com/a/37426929/5881346
 RUN sed -i "s/httpredir.debian.org/debian.uchicago.edu/" /etc/apt/sources.list && \
+    apt-get clean && \
     apt-get update --allow-releaseinfo-change && \
     # Needed by lightGBM (GPU build)
     # https://lightgbm.readthedocs.io/en/latest/GPU-Tutorial.html#build-lightgbm