Delete untagged images (#151)

This change to the pipline deletes untagged / overwritten tags once a
week to keep the memory consumption in the GH registry reasonable.

.github/workflows/maintenance.yaml

@@ -0,0 +1,18 @@
+name: Auto Maintenance Cycle
+
+on:
+  schedule:
+  - cron: 30 7 * * 1
+
+jobs:
+  delete_all_untagged:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        name: [ingest, data-transformation, web-server]
+    steps:
+    - name: purge packages
+      uses: dylanratcliffe/delete-untagged-containers@main
+      with:
+        package_name: ${{ github.event.repository.name }}/${{ matrix.name }}
+        token: ${{ secrets.GITHUB_TOKEN }}