It's going to be released tomorrow, so let's use the latest Debian release
as the base image while we are at it. Also run always `apt upgrade` to upgrade
any outdated packages (if any).
Previously, the https://github.com/Icinga/docker-icinga2 repository was
used to build the Docker images for Icinga 2. However, due to its
various design flaws, the resulted images had limited usability and
required a lot of manual tweaking to make something useful out of them.
This commit now follows our new principles of building Docker images
from the Icinga DB repository, and replaces the old separate repository
with this one. It makes use of the newest Docker BuildKit features to
build the images in a more efficient way, while also granting users full
flexibility to easily extend or modify the images as they see fit
without any issues.
