Add cleanup pipeline to remove unusable images (#51)

This PR adds a weekly cleanup job to remove untagged, orphaned, ghost,
and partial images.

Per the description at https://github.com/dataaxiom/ghcr-cleanup-action:
* delete-untagged = Delete all untagged images
* delete-ghost-images = Delete multi-architecture images where all
underlying platform images are missing
* delete-partial-images = Delete multi-architecture images where some
(but not all) underlying platform images are missing
* delete-orphaned-images = Delete tagged images which have no parent
(e.g. referrers and cosign tags missing their parent)

Author: bthomee

.github/workflows/cleanup.yml

@@ -0,0 +1,45 @@
+name: Docker image cleanup
+
+on:
+  pull_request:
+    paths:
+      - .github/workflows/cleanup.yml
+  push:
+    branches:
+      - main
+    paths:
+      - .github/workflows/cleanup.yml
+  schedule:
+    - cron: "42 2 * * 0" # Run every Sunday at 02:42 UTC.
+  workflow_dispatch:
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    permissions:
+      packages: write
+    strategy:
+        fail-fast: false
+        matrix:
+          image:
+            - debian-bullseye
+            - debian-bookworm
+            - gcc
+            - rhel-9.4
+            - rhel-9.6
+            - tools-rippled-clang-format
+            - tools-rippled-documentation
+            - tools-rippled-pre-commit
+            - tools-rippled-prettier
+            - ubuntu-jammy
+            - ubuntu-noble
+    steps:
+      - name: Cleanup images in the GitHub Container Registry
+        uses: dataaxiom/ghcr-cleanup-action@cd0cdb900b5dbf3a6f2cc869f0dbb0b8211f50c4 # v1.0.16
+        with:
+          delete-ghost-images: true
+          delete-orphaned-images: true
+          delete-partial-images: true
+          delete-untagged: true
+          dry-run: ${{ github.event_name == 'pull_request' }}
+          package: ci/${{ matrix.image }}