Pin the `cmake-format` version to `0.6.7`

See merge request docker-images/gr-build-images!4

This commit belongs to the changes of
[GR MR !339](https://iffgit.fz-juelich.de/Scientific-IT-Systems/gr/merge_requests/339).

Add `six` for `cmake_format`

See merge request docker-images/gr-build-images!3

`cmake_format` needs `six` but it is not declared as a dependency.
Therefore, install `python3-six` explicitly.

Add missing `file` command to the code-style-check image

See merge request docker-images/gr-build-images!2

The last commit added the `no-install-recommends` option to apt to
create a smaller image but the needed `file` command was removed. This
commit adds `file` again.

Add `cmake_format` to the code style check Dockerfile

See merge request docker-images/gr-build-images!1

These changes are needed for
[MR !317](https://iffgit.fz-juelich.de/Scientific-IT-Systems/gr/merge_requests/317).