SOLR-15075: Remove docker gradle artifact

Gradle is currently picking up the dockerBuild task in "assemble",
because of the build artifact.
diff --git a/solr/docker/build.gradle b/solr/docker/build.gradle
index 723f664..6b7e214 100644
--- a/solr/docker/build.gradle
+++ b/solr/docker/build.gradle
@@ -200,10 +200,4 @@
 // 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)
-  }
 }
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