Improve Developer Experience (updated README, new Rake tasks, etc)

[tknerr]

Improves the developer experience:

• refactor Rake tasks and tests so that you can now build selected platforms only, e.g.
  • rake <task> PLATFORM=all will build all os / version combinations
  • rake <task> PLATFORM=ubuntu will build all ubuntu versions
  • rake <task> PLATFORM=ubuntu-22.04 will build only the ubuntu 22.04 version
• added new (interactive) rake tasks for publishing to dockerhub and vagrantcloud:
  • rake publish:docker:base_images will publish the selected Docker base images to dockerhub
  • rake publish:vagrant:baseboxes will publish the selected Vagrant baseboxes to vagrantcloud
• added missing development section to README, describing the usage of the rake tasks for development