The sdist tarball produced by poetry build does not include
all files and skipped symlinks in test/data. This caused the
unit test run to fail if called from within that sources
The new optional metadata file image/exclude_files.yaml can
be placed inside of the local image root tree. At creation time of
the image binary the file contents are used to extend the default
exclude list with additional information. The structure of the
file must be as follows:
```yaml
exclude:
- exclude-name-used-in-rsync
```
