So I can confirm Solr does use "java.io.tmpdir". Unfortunately, it's not really configurable. Forcing a custom "java.io.tmpdir" in XWiki is technically possible (I assume we can change the value of any system property by maybe I'm wrong, to be tested), but it's not very clean to force that by default since it will impact everything in the JVM, so the application server internal stuff and any other application.

I'm afraid the best we can do at XWiki level is to provide an option (disabled by default) to set "java.io.tmpdir" to the configured XWiki temporary folder. And then to enable it in the XJetty package and in the Docker image.

WDYT Vincent Massol ?

Were you able to force setting a custom -Djava.io.tmpdir (in the tomcat script in the docker image, for example), to at least confirm it fixes your problem and working in that direction to do the trick for you?