Add missing apt-get clean in Ubuntu base image

TrivialFix

Change-Id: Ibd4ba9dbe2cadb304e027b60d5b7378416719d12

docker/base/Dockerfile.j2

@@ -208,7 +208,9 @@ ENV DEBIAN_FRONTEND noninteractive
 COPY dpkg_reducing_disk_footprint /etc/dpkg/dpkg.cfg.d/dpkg_reducing_disk_footprint
 
 # Need apt-transport-https BEFORE we replace sources.list or apt-get update wont work!
-RUN apt-get update && apt-get -y install --no-install-recommends apt-transport-https ca-certificates
+RUN apt-get update \
+    && apt-get -y install --no-install-recommends apt-transport-https ca-certificates \
+    && apt-get clean
 
 COPY sources.list.{{ base_distro }} /etc/apt/sources.list
 COPY apt_preferences.{{ base_distro }} /etc/apt/preferences