[pica-db-backup-rotation] Update Dockerfile

/etc/crontab does not exist in Alpine. No idea why the `touch 
/crontab.conf` was added.

pica-db-backup-rotation/Dockerfile

@@ -8,4 +8,4 @@ CMD /start_rotation.py && \
     crond && \
     touch /crontab.conf && \
     crontab /crontab.conf && \
-    tail -f /etc/crontab
+    cat /crontab.conf