ensure package cache is not stored

Dockerfile

@@ -9,7 +9,7 @@ ENV DEBIAN_FRONTEND noninteractive
 RUN echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections
 
 # install available updates
-RUN apt-get update && apt-get --assume-yes upgrade
+RUN apt-get update && apt-get --assume-yes upgrade && apt-get clean
 
 # support https repositories
 RUN apt-get --assume-yes install \
@@ -18,7 +18,8 @@ RUN apt-get --assume-yes install \
       bash-completion \
       curl \
       lsb-release \
-      wget
+      wget && \
+    apt-get clean
 
 # Add other repos
 RUN wget http://software.ligo.org/lscsoft/debian/pool/contrib/l/lscsoft-archive-keyring/lscsoft-archive-keyring_2016.06.20-2_all.deb && \
@@ -27,6 +28,3 @@ RUN wget http://software.ligo.org/lscsoft/debian/pool/contrib/l/lscsoft-archive-
 
 RUN echo "deb http://software.ligo.org/gridtools/debian buster main" > /etc/apt/sources.list.d/gridtools.list && \
     echo "deb [trusted=yes] https://galahad.aei.mpg.de/lsc-amd64-buster ./" > /etc/apt/sources.list.d/lscsoft.list
-
-# cleanup
-RUN apt-get clean
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