Start buster branch

Dockerfile

-FROM registry.icinga.com/build-docker/raspbian-base/stretch
+FROM registry.icinga.com/build-docker/raspbian-base/buster
 
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update \
  && apt-get upgrade -y \
- && rm -rf /var/lib/apt/lists/* # 20190220
+ && rm -rf /var/lib/apt/lists/* # 20190516
 
 RUN apt-get update \
  && apt-get install -y \