Currently the local (and test-official) docker images built off of 9x (main) are 1.5-2x the size of the docker images created for 8.x releases.
You can see this in the official nightly builds docker image, when you click on the image tag.
The reason for this is we change the permission of each Solr file/directory that we have either 1) just downloaded (in the official docker image) or 2) just copied (in the local docker image), in this line: https://github.com/apache/solr/blob/main/solr/docker/templates/Dockerfile.body.template#L34
One easy fix is to do this step in the same download step in the official docker file, so that no additional space is used up for that layer. However, since the copy task does not allow for changing permissions at the same time, local docker builds will continue to have the same size issues.