- Bump version: 9.17.30 → 9.17.31

- Update the documentation regarding vagrant boxes
Extend the documentation on how to build VirtualBox vagrant boxes
All this was done by Dan Čermák

- Refactor incremental changelog update
The creation of the package changelog is based on a reference file.
However that reference file contained log information in a specific
timezone which requires to hardcode the region of that timezone
in the code to correctly run date/time calculations. This can
be done better from a conceptual point of view. This patch changes
the handling in a way that the reference file is a git log
excerpt including the dates as git log lists them. The dates
contains complete numeric time/date/zone information and can be
used for calculations. The changelog helper tool converts the
result data to match the requirements of rpm changelog files
and prints the time/date information localized to the callers
timezone or as UTC if the --utc switch is given. By default the
user local timezone settings applies. That way the setup of
the local timezone is immaterial to the changelog processor
and the workaround in the gitlab-ci rpm stage can be deleted
too.

- Fix derived docker images build
This commit fixes the derived docker images when the base image is a
compressed file. After the refactor in #998 the decompression of the
base image and the skopeo call to import the decompressed image happened
in absolutely independent scopes. NamedTemporaryFile python class by
default deletes the created temporary file when the class instance is