This patch replaces the container-strucuture-tests for the Zeek
container with a BTest-driven approach to provide a more familiar
experience. In addtion to `python3` for `btest` we also rely on `make`
being available in Github-hosted runners which at least currently seems
to be hold (probably pulled in as a dependency of CMake which is
officially included, see
https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md).
This patch adds a minimal Zeek environment packaged as a container.
Since this is intended both as a base layer for other images and as a
quick way to explore Zeek we install only zeek and zkg as basic
functionality.
Closes#1625.
