According to the Scribuntu Extension page, Scribuntu is included with Mediawiki 1.34. This is the Mediawiki version used in the latest Wikibase Docker image. However, Scribuntu is apparently not included.
It is unclear how to manually add Scribuntu to the Docker image. The way that worked previously—using a volume to map the extension's files into the container—has apparently become incompatible with Mediawiki 1.34.