Stop Renovate holding docker image updates on a check that can never pass (#24848)

ghcr.io and registry.ddbuild.io do not return release timestamps, and since Renovate
42 a missing timestamp counts as never having aged, so the 7-day
renovate/stability-days check on those images stays pending forever. Scope
minimumReleaseAgeBehaviour to timestamp-optional for that group, which keeps the
age check wherever a registry does publish timestamps.

Also enable the dependency dashboard, so updates held back by the age check are
visible instead of just absent.

Author: AAraKKe

renovate.json

@@ -1,5 +1,6 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "dependencyDashboard": true,
   "enabledManagers": [
     "github-actions",
     "custom.regex"
@@ -160,6 +161,7 @@
       "minimumReleaseAge": "7 days"
     },
     {
+      "description": "Neither ghcr.io nor registry.ddbuild.io return release timestamps, and Renovate treats a missing timestamp as never having aged. Without timestamp-optional these updates sit with a permanently pending renovate/stability-days check.",
       "matchManagers": [
         "custom.regex"
       ],
@@ -174,7 +176,8 @@
         "renovate/docker-images",
         "qa/skip-qa"
       ],
-      "minimumReleaseAge": "7 days"
+      "minimumReleaseAge": "7 days",
+      "minimumReleaseAgeBehaviour": "timestamp-optional"
     }
   ]
 }