Update Dockerfile

Signed-off-by: Kevin H. Luu <kevin@anyscale.com>

docker/Dockerfile

@@ -82,6 +82,7 @@ ARG GET_PIP_URL
 # Install Python and other dependencies
 RUN echo 'tzdata tzdata/Areas select America' | debconf-set-selections \
     && echo 'tzdata tzdata/Zones/America select Los_Angeles' | debconf-set-selections \
+    && apt-get clean -y \
     && apt-get update -y \
     && apt-get install -y ccache software-properties-common git curl sudo \
     && if [ ! -z ${DEADSNAKES_MIRROR_URL} ] ; then \