[DBBackup] Switch to Debian image for PG12 client

pica-db-backup/CHANGELOG.md

 ## Version 1.3
 
+* Switch from Python image to Bullseye image
 * Update for PG12+
 * Default cron hour at 03:00AM

pica-db-backup/Dockerfile

-FROM python:3-slim-buster
+FROM debian:bullseye-slim
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     iputils-ping \
     postgresql-client-12 \
+    python3 \
     default-mysql-client \
     mongo-tools \
     cron && \