ci: Retention policy for docker prerelease tags

Delete any `-dev.` prerelease tags older than a week.

.github/workflows/docker-retention.yml

@@ -0,0 +1,25 @@
+# yaml-language-server: $schema=https://json.schemastore.org/github-workflow.json
+name: Docker Retention
+
+on:
+  workflow_dispatch: # TEMP
+  schedule:
+    - cron: '0 0 * * *'
+
+jobs:
+  retention:
+    name: Enforce Retention Policy
+    runs-on: ubuntu-latest
+    steps:
+      - name: Delete old pre-release images
+        uses: snok/container-retention-policy@v2
+        with:
+          image-names: recyclarr
+          filter-tags: '*-dev.*'
+          cut-off: One week ago CST
+          keep-at-least: 1
+          account-type: org
+          org-name: recyclarr
+          token: ${{ secrets.GITHUB_TOKEN }}
+          token-type: github-token
+          dry-run: true # TEMP