SOLR-15075: Remove docker gradle artifact

Gradle is currently picking up the dockerBuild task in "assemble",
because of the build artifact.

solr/docker/build.gradle

@@ -200,10 +200,4 @@ task dockerPush(dependsOn: tasks.dockerTag) {
 // One task to build and tag a Solr docker image
 task docker {
   dependsOn tasks.dockerBuild, tasks.dockerTag
-}
-
-artifacts {
-  dockerImage(tasks.dockerBuild.outputs.files.singleFile) {
-    builtBy(tasks.dockerBuild)
-  }
 }