Step: mount
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Mount a filesystem.

Step keys:

* `mount` &mdash; REQUIRED; tag of filesystem to mount.

* `dirname` &mdash; OPTIONAL; the mount point.

* `mount-on` &mdash; OPTIONAL; tag of already mounted filesystem in
  image. (FIXME: this may be wrong?)

* `zerofree` &mdash; OPTIONAL; Boolean flag controlling whether or not to run
  the `zerofree` utility on the filesystem after unmounting it at the end of
  the build process. Defaults to `true`

Example (in the .vmdb file):

    - mount: root
