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

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

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.

* `install_keyring` &mdash; OPTIONAL; if set to `yes`, the gpg keyring
  specified by the `keyring` key will be installed in the image for use when
  installing packages from non-official Debian repositories.

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

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

* `include` &mdash; OPTIONAL; a list of additional packages for
  debootstrap to install.

* `tls_ca_certs` &mdash; OPTIONAL; a list of paths to TLS Certificate
  Authority (CA) cert files to install in the image after the debootstrap
  process has completed. This allows the use of package repositories with
  HTTPS transports that use TLS certificates issued by private CAs.
  Note that the CA cert files being installed must have a `.crt` suffix
  in order to be used.

Example (in the .vmdb file):

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