Adds a new --localversion option to configure.
The value of localversion becomes part of the Zeek version string.
Docker builds allow an environment variable (LOCALVERSION) to set
the localversion string.
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.
