0.0.6

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