[pica-db-backup-rotation] Update Dockerfile

* Try to prevent the container from restarting all the time by adding 
`-f` to `crond`
* `touch /crontab.conf` not needed
* `cat /crontab.conf` not needed because `start-rotation.py` already 
prints the config

pica-db-backup-rotation/Dockerfile

@@ -5,7 +5,5 @@ RUN pip install --no-cache-dir "rotate-backups<9" "python-crontab<2.5"
 COPY start_rotation.py /start_rotation.py
 
 CMD /start_rotation.py && \
-    crond && \
-    touch /crontab.conf && \
-    crontab /crontab.conf && \
-    cat /crontab.conf
+    crond -f && \
+    crontab /crontab.conf