Step: `qemu-debootstrap`
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Install packages using apt, which needs to already have been
installed, for a different architecture than the host where vmdb2 is
being run. For example, for building an image for a Raspberry Pi on an
Intel PC.

Step keys:

* `qemu-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; OPTIONAL; which Debian mirror to use.

* `arch` &mdash; REQUIRED; the foreign architecture touse.

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

Example (in the .vmdb file):

    - qemu-debootstrap: stretch
      target: root
      mirror: http://mirror.example.com/debian
      arch: arm64
      variant: buildd
