Step: cache-rootfs
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Create a tarball of the root filesystem in the image.

Step keys:

* `cache-rootfs` &mdash; REQUIRED; tag of root filesystem on image.
* `force` &mdash; OPTIONAL; boolean that enables overwriting of an existing rootfs
    tarball by `vmdb2`, allowing the tarball to be used as both a build input and a
    build output. This can be useful in multi-stage build chains where a "common base
    OS" rootfs filesystem is populated and packaged as a rootfs tarball by `vmdb2`, and
    this tarball is consumed by multiple downstream `vmdb2` builds that extract the
    tarball into disk images that have different partition layouts and/or filesystems.

Example (in the .vmdb file):

    # typical use
    - cache-rootfs: root
      unless: rootfs_unpacked

    # create a rootfs tarball output at the end of a build process
    - cache-rootfs: root
      force: true
