Overview of changes
- Add label for tagging, not tied to the fedora-toolbox name
- Add short variants for various options in 'create' and 'enter'
- Ensure that names of toolbox containers don't have a colon
- Enable Travis
- Fix miscellaneous issues pointed out by https://www.shellcheck.net/
- Give access to Kerberos if KCM credential caches are being used
- Improve the onboarding experience
- Reduce the sizes of the images by removing temporary files created by DNF
- Use a lighter entry point than /bin/sh