Checkmk Werk 12277: Docker container: Update base image to Debian buster

ID: 12277 Title: Docker container: Update base image to Debian buster Component: Core & setup Level: 2 Class: Security fix Version: 2.1.0i1 The Checkmk docker container image was previously based on the debian:stretch-slim image. The base image has now been updated to debian:buster-slim. If you build the container images on your own, based on the Dockerfile from our git, you will now have to use the Checkmk packages for Debian buster instead of the stretch packages.