Overview of changes
- Add a list command
- Drop the "fedora" prefix and rename the project as just "toolbox"
- Fix typos pointed out by https://www.shellcheck.net/
- Lots of Bash-isms removed for POSIX correctness
- Make --container and --image command-specific options
- Make it work inside the toolbox container itself
- Shorten the prefix for debug and error messages
- Use the host's PID namespace for the toolbox container
- Use the standard error output for error messages