After setting up the docker environment with Wikibase, Special:EntityData (i.e. the equivalent of something like Special:EntityData/Q666.json on wikidata) didn't work as expected. It contained gzip compressed json, but was missing the necessary Content-encoding header, so it displayed gibberish when viewed in the browser and it broke Item/Property pages, which make client-side requests to Special:EntityData. The compression worked fine for other types of requests, e.g. js/css assets loaded via ResourceLoader were gzipped and contained the respective headers. Adding header( 'content-encoding: gzip' ); e.g. to LocalSettings.php"fixed" the problem for Special:EntityData, but that's obviously not a proper solution.