Update container image cleanup GitHub actions

Switch to dataaxiom/ghcr-cleanup-action which treats multi-arch images
correctly and refactor the cleanup jobs so that we delete untagged
versions every time after building a new image and delete PR tags once
the PR is closed (merged or unmerged).

.github/workflows/gitlab-container-image-cleanup-pr-tag.yml

@@ -0,0 +1,21 @@
+---
+name: Delete closed PR container image tag
+
+"on":
+  pull_request:
+    types:
+      - closed
+
+jobs:
+  cleanup-pr-tag:
+    uses: ubuntu-latest
+    steps:
+      - name: Set image version for PR to branch name
+        run: echo "VERSION=${GITHUB_HEAD_REF//\//-}" >> ${GITHUB_ENV}
+
+      - name: Delete PR container image tag
+        uses: dataaxiom/ghcr-cleanup-action@v1
+        with:
+          tags: ${{ env.VERSION }}
+          package: ${{ github.event.repository.name }}/gitlab
+          token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/gitlab-container-image.yml

@@ -46,3 +46,15 @@ jobs:
           platforms: linux/amd64,linux/arm64
           push: true
           tags: 'ghcr.io/${{ env.IMAGE }}:${{ env.VERSION }}'
+
+      - name: Delete untagged container images
+        # We always delete all untagged container images after building an
+        # image. This way, there should never be stale untagged images laying
+        # around in the registry. In combination with the workflow that
+        # deletes PR tags after the PR is closed we should be able to keep the
+        # container image registry size in check.
+        uses: dataaxiom/ghcr-cleanup-action@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          package: ${{ github.event.repository.name }}/gitlab
+          validate: true