Use Debian buster as the base image instead of stretch

Updates the base image for ruby_docker and debian_packer from stretch to buster.

Previous MR (jessie to stretch): !101 (merged)

After the MR

We would have only Debian stretch in the debian_9 image:

$ git grep stretch
docker/Dockerfile_debian_9.erb:FROM debian:stretch as builder
docker/Dockerfile_debian_9.erb:FROM debian:stretch

Closes #35 (closed) #36 (closed)