[DBBackupRotation] Switch to a Python image

pica-db-backup-rotation/Dockerfile

-FROM debian:buster-slim
+FROM python:3-slim-buster
 
 RUN apt-get update \
     && apt-get -y upgrade \
-    && apt-get -y install cron python3 python3-pip \
+    && apt-get -y install cron
     && pip3 install rotate-backups python-crontab \
     && rm -rf /var/lib/apt/lists/*
 
-VOLUME  ["/backup"]
-VOLUME  ["/config"]
-
 COPY start_rotation.py start_rotation.py
 
 RUN touch /crontab.conf