[pica-db-backup-rotation] Fix CVE-2020-8492

CVE-2020-8492 reported by Clair in previous build: 
https://gitlab.utc.fr/picasoft/projets/dockerfiles/-/jobs/883005

This comes from 'libpython2.7', which is one of many unnecessary 
dependencies that get installed with 'cron' dy default

pica-db-backup-rotation/Dockerfile

@@ -2,7 +2,7 @@ FROM python:3-slim-buster
 
 RUN apt-get update \
     && apt-get -y upgrade \
-    && apt-get -y install cron \
+    && apt-get -y install --no-install-recommends cron \
     && pip3 install rotate-backups python-crontab \
     && rm -rf /var/lib/apt/lists/*