speedup docker builds with a cache

.github/workflows/docker_cache-cleaner.yml

@@ -0,0 +1,25 @@
+name: ci-package-cleaner
+
+on:
+
+  workflow_dispatch: # manual option
+
+  schedule:
+    - cron: '15 22 * * 1' # every Monday 10.15pm UTC (~11.15am Tuesday NZT)
+
+jobs:
+
+  package-cleaner:
+    name: package-cleaner
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    permissions:
+      packages: write
+    steps:
+
+      - uses: actions/delete-package-versions@v4
+        with: 
+          package-name: pi.alert
+          package-type: container
+          min-versions-to-keep: 0
+          delete-only-untagged-versions: true