Add `exportDockerImages` lifecycle task for exporting docker tarballs (#91571)

We need exported Docker artifacts for the release process. Add a
lifecycle task to generate these artifacts.

.ci/jobs.t/elastic+elasticsearch+periodic+dra-snapshots.yml

@@ -29,7 +29,7 @@
                 -Ddra.artifacts.dependency.ml-cpp=${ML_CPP_BUILD_ID} \
                 -Dcsv=$WORKSPACE/build/distributions/dependencies-${ES_VERSION}-SNAPSHOT.csv \
                 buildReleaseArtifacts \
-                :distribution:docker:assemble \
+                exportDockerImages \
                 :distribution:generateDependenciesReport
 
             unset VAULT_TOKEN

distribution/docker/build.gradle

@@ -472,6 +472,8 @@ boolean isArchitectureSupported(Architecture architecture) {
   return serviceProvider.get().dockerAvailability.supportedArchitectures().contains(architecture)
 }
 
+def exportDockerImages = tasks.register("exportDockerImages")
+
 /*
  * The export subprojects write out the generated Docker images to disk, so
  * that they can be easily reloaded, for example into a VM for distribution testing
@@ -504,7 +506,7 @@ subprojects { Project subProject ->
     final String buildTaskName = taskName('build', architecture, base, 'DockerImage')
     final String tarFile = "${parent.projectDir}/build/${artifactName}_${VersionProperties.elasticsearch}.${extension}"
 
-    tasks.register(exportTaskName, LoggedExec) {
+    def exportTask = tasks.register(exportTaskName, LoggedExec) {
       inputs.file("${parent.projectDir}/build/markers/${buildTaskName}.marker")
       executable = 'docker'
       outputs.file(tarFile)
@@ -517,6 +519,10 @@ subprojects { Project subProject ->
       onlyIf { isArchitectureSupported(architecture) }
     }
 
+    exportDockerImages.configure {
+      dependsOn exportTask
+    }
+
     artifacts.add('default', file(tarFile)) {
       type 'tar'
       name artifactName