Fix generating docker exports by triggering assemble (elastic#133475)

The kibana team relies on using
`:distribution:docker:docker-export:assemble` which has been broken
after the gradle 9.0 update due to the (by now known) change in
behaviour of not tying default artifacts to the assemble task anymore.

The recommended fix is, setting this up explicitly.

Author: breskeby

distribution/docker/build.gradle

@@ -588,6 +588,11 @@ subprojects { Project subProject ->
       dependsOn compressExportTask
     }
 
+    tasks.named('assemble').configure {
+      dependsOn exportTask
+    }
+
+    // deprecated here for backwards compatibility of DistroTestPlugin and DistributionDownloadPlugin
     artifacts.add('default', file(tarFile)) {
       type = 'tar'
       name = artifactName