Step: virtual-filesystems
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Mount the usual Linux virtual filesystems in the chroot:

* `/proc`
* `/dev`
* `/dev/pts`
* `/dev/shm`
* `/run`
* `/run/lock`
* `/sys`

They will be automatically unmounted at the end.

Often, the virtual filesystems are unnecessary, but some Debian
packages won't install without them. The grub boot loader needs them
as well, but mounts what it needs itself, if necessary.

Step keys:

* `virtual-filesystems` — REQUIRED; value is the tag of
  the root filesystem.

Example (in the .vmdb file):

~~~yaml
- virtual-filesystem: rootfs
~~~