Revert "Temporarily disable intermediate registry prune"

This reverts commit 95f0c32a61.

We add a bit more code on top of the revert to capture output in a log
file (pruning is very verbose) and rotate 30 daily copies of the log
file.

Change-Id: I1778989d676d316537cf135b8e14a255434091b4

playbooks/roles/registry/tasks/main.yaml

@@ -75,11 +75,25 @@
   shell:
     cmd: docker image prune -f
 
-# Temporarily disable to aid debug of mysteriously absent blobs
-# -corvus 2019-10-09
-# - name: Install cron to garbage collect the registry daily
-#   cron:
-#     name: "docker registry garbage-collect"
-#     minute: "0"
-#     hour: "0"
-#     job: "/usr/bin/docker exec registrydocker_registry_1 zuul-registry prune"
+- name: Create registry prune log output directory
+  file:
+    path: /var/log/registry-prune
+    state: directory
+    owner: root
+    group: root
+    mode: '0755'
+
+- name: Install cron to garbage collect the registry daily
+  cron:
+    name: "docker registry garbage-collect"
+    minute: "0"
+    hour: "0"
+    job: "/usr/bin/docker exec registrydocker_registry_1 zuul-registry prune >> /var/log/registry-prune/prune.log 2>&1"
+
+- name: Install logrotate rules
+  include_role:
+    name: logrotate
+  vars:
+    logrotate_rotate: 30
+    logrotate_file_name: '/var/log/registry-prune/prune.log'
+    logrotate_frequency: 'daily'