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

Step keys:

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

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

* `mirror` — REQUIRED; which Debian mirror to use

* `keyring` — 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` — OPTIONAL; the foreign architecture to use.

* `variant` — 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