Step: debootstrap
-----------------------------------------------------------------------------

Create a directory tree with a basic Debian installation. This does
not include a boot loader.

A side effect of this step is that `apt-get update` is **always** run,
even if the step is otherwise skipped using `unless`.

Step keys:

* `debootstrap` &mdash; REQUIRED; value is the codename of the Debian
  release to install: `stretch`, `buster`, etc.

* `target` &mdash; REQUIRED; value is the tag for the root filesystem.

* `mirror` &mdash; REQUIRED; which Debian mirror to use

* `keyring` &mdash; OPTIONAL; which gpg keyring to use to verify the
  packages. This is useful when using a non-official Debian repository
  (e.g. Raspbian) as by default debootstrap will use the keys provided
  by the "debian-archive-keyring" package.

* `arch` &mdash; OPTIONAL; the foreign architecture to use.

* `variant` &mdash; OPTIONAL; the variant for debootstrap.

Example (in the .vmdb file):

    - debootstrap: buster
      target: root
      mirror: http://mirror.example.com/debian
      keyring: /etc/apt/trusted.gpg
