CI: clean up Docker/Podman build artifacts after APK build

build-apk.sh always builds with --no-cache, which piles up images,
build cache and containers on the runner over time. Add a cleanup
step (if: always(), runs even on failure) that prunes everything via
'docker system prune -af --volumes' or the Podman equivalent, so
self-hosted runners don't fill up their disk.

.github/workflows/ci.yml

@@ -38,3 +38,15 @@ jobs:
         with:
           name: tiptoi-sync-apk
           path: apk-output/tiptoi-sync.apk
+
+      # Läuft immer, auch wenn der Build fehlschlägt, damit --no-cache-Images/
+      # Build-Cache/Container nicht den Runner-Datenträger zumüllen
+      # (insbesondere relevant für selbst-gehostete/Gitea-Runner).
+      - name: Clean up Docker/Podman build artifacts
+        if: always()
+        run: |
+          if command -v podman > /dev/null 2>&1; then
+            podman system prune -af --volumes
+          elif command -v docker > /dev/null 2>&1; then
+            docker system prune -af --volumes
+          fi