Improve out-of-box experience with single-instance container

[Martchus]

• Give the container a name for easier reference in further explanations
• Add KVM support; this is very useful as many jobs in our instance seem
  to use QEMUCPU=host and would therefore otherwise fail because this
  setting requires KVM support
• Provide an example command for cloning a job and for making an API query;
  otherwise it isn't clear how to proceed with a containerized setup like
  that after exploring the web UI
• Explain how to provide test code in a containerized setup like this
• See https://progress.opensuse.org/issues/153499

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching docs/*.asciidoc:

• Consider generating documentation locally to verify it is rendered correctly using tools/generate-docs

This is an automatically generated QA checklist based on modified files.

On second thought the user story is unclear

[Martchus]

I shortened this to the parts we hopefully can all agree on.