Delete unversioned images after 6 months (#1109)

Standardized with `docker-rcran`: Kaggle/docker-rcran#57 (comment)

Author: rosbo

Jenkinsfile

@@ -167,8 +167,8 @@ pipeline {
       steps {
         sh '''#!/bin/bash
           set -exo pipefail
-          gcloud container images list-tags gcr.io/kaggle-images/python --filter="NOT tags:v* AND timestamp.datetime < -P9M" --format='get(digest)' --limit 100 | xargs -I {} gcloud container images delete gcr.io/kaggle-images/python@{} --quiet --force-delete-tags
-          gcloud container images list-tags gcr.io/kaggle-private-byod/python --filter="NOT tags:v* AND timestamp.datetime < -P9M" --format='get(digest)' --limit 100 | xargs -I {} gcloud container images delete gcr.io/kaggle-private-byod/python@{} --quiet --force-delete-tags
+          gcloud container images list-tags gcr.io/kaggle-images/python --filter="NOT tags:v* AND timestamp.datetime < -P6M" --format='get(digest)' --limit 100 | xargs -I {} gcloud container images delete gcr.io/kaggle-images/python@{} --quiet --force-delete-tags
+          gcloud container images list-tags gcr.io/kaggle-private-byod/python --filter="NOT tags:v* AND timestamp.datetime < -P6M" --format='get(digest)' --limit 100 | xargs -I {} gcloud container images delete gcr.io/kaggle-private-byod/python@{} --quiet --force-delete-tags
         '''
       }
     }