The `koji-osbuild` plugin supports additional formats for the `repo`
property since v4 [1]. Specifically, a repo can be specified as a
dictionary with `baseurl` key and `package_sets` list containing
specific package set names, that the repository should be used for.
Extend the configuration schema to reflect the plugin change.
Extend the documentation to cover the new repository format.
Extend an existing unit test to specify additional repository using the
added format.
[1] https://github.com/osbuild/koji-osbuild/pull/82
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
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