Clear cache after python install

Author: james-toussaint

cloud-computing/gramine/Dockerfile

@@ -1,7 +1,7 @@
 # FIXME: use tagged version when released
 FROM docker-regis.iex.ec/iexec-graphene-base:0.8.0
 
-RUN  apt-get install -y python3 \
+RUN apt-get install -y python3 && rm -rf /var/lib/apt/lists/* \
      && pip3 install pyfiglet
 
 ARG SOURCE_DIR=src