Make the build tool more dev-friendly and packager-friendly

This pull request improves the build tool to make it easier to develop, test and package Docker.

* Run the tests inside a docker container using the -privileged flag
* Divide the build scripts into smaller, more maintainable parts
* Vendor dependencies to make some distro packager's lives easier
* Add friendly notes to distro packagers in hack/PACKAGERS.md
* Document a rapid development workflow with bind-mounted source
* Include the output of the tests as part of a release, for auditing