Use KUBERNETES_VERSION consistently

[twz123]

Description

There were different environment variables in use to transport the current Kubernetes version. Unify that by replacing all KUBE_VERSION usages with KUBERNETES_VERSION.

Don't embed the Kuberntes version in the docs helper Docker container. It's not used during build time. It's required during runtime, though. Hence pass this as an -e parameter to the docker run command instead. Let the Docker container build also depend on the embedded-bins Makefile variables, as the Alpine version is defined over there.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings