Container fixups

[troglobit]

Description

• Update container docs, again
• Massive refactor and simplification of hostnamefmt(), as a follow-up to

  716

Ping @rical

Checklist

Tick relevant boxes, this PR is-a or has-a:

• [ ] Bugfix
  • [ ] Regression tests
  • [ ] ChangeLog updates (for next release)
• [ ] Feature
  • [ ] YANG model change => revision updated?
  • [ ] Regression tests added?
  • [ ] ChangeLog updates (for next release)
  • [ ] Documentation added?
• [ ] Test changes
  • [ ] Checked in changed Readme.adoc (make test-spec)
  • [ ] Added new test to group Readme.adoc and yaml file
• [ ] Code style update (formatting, renaming)
• [x] Refactoring (please detail in commit messages)
• [ ] Build related changes
• [x] Documentation content changes
• [ ] Other (please describe):

[rical]

Great initiative refactoring this! Unfortunately I don't have time to review the actual code right now, but knowing its refactored will make me sleep better ;-)