Refactor/rewrite meta task

[ahayzen]

Consider various changes

• Only via a declaration in a folder
• Automatically mount in /nix so that developers tooling can be "layered" into the container for editors etc
• Move away from shell scripts
• Improve CLI

Target

folderbox  // auto launches/enters
folderbox --rebuild  // forces rebuild of the image

Structure

~/.config/folderbox/settings.ini  // global settings for eg sandbox or custom args
~/.local/share/folderbox/persist/<container>/  // persistent storage for container
~/.local/share/folderbox/snippets/  // reusable snippets

CWD
  .folderbox/
    container.in  // Dockerfile but with replacement of known $ paths
    settings.ini  // custom settings for eg sandbox or custom args