Clean /tmp after installing with pip

latest/Dockerfile

@@ -24,6 +24,7 @@ RUN mkdir -p /auth && \
     ln -s /auth/htgroups /trac/.htgroups && \
     mkdir -p /home/csa/ && \
     ln -s /trac /home/csa/.trac && \
+    rm -rf /tmp/.cache/ && \
     chmod g=u /etc/passwd
 
 COPY docker-entrypoint.sh /