Step: create-file
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Create an empty file in the target filesystem.

Step keys:

* `create-file` &mdash; REQUIRED; the full (starting from the new
  filesystem root) path name of the file to create. It will *not*
  create any directories; if they need to be created, please use
  `create-dir` first.
* `contents` &mdash; REQUIRED; the contents to be written to the
  generated file.
* `perm` &mdash; OPTIONAL; the numeric (octal) representation of the
  file's permissions. Defaults to 0644.
* `uid` &mdash; OPTIONAL; the numeric user ID of the file's
  owner. Defaults to 0 (root).
* `gid` &mdash; OPTIONAL; the numeric user ID of the file's
  group. Defaults to 0 (root).
