Fix gradle workflow for non-signed official images

In SOLR-16748, the Official Solr dockerfile was cleaned up, and support for non-signed artifacts was added by checking if the SOLR_DOWNLOAD_SERVER contained "apache.org".

This means that we can support building the official Dockerfile without signing artifacts. And this is currently the case. However, when building the release without signing ({{ gradle assembleRelease -Psign=false -x signJarsPublication }}), we should include the official dockerfile, because the release distribution docker/ folder really should only have the official dockerfiles, not the local dockerfiles. This is a remnant of the old requirements for signing.

Also now we should be able to test the official Dockerfiles when they aren't signed. This will allow us to test them on Jenkins and for non-committers to test, because they can't sign without being in the Solr KEYS file.