ci: preserve Docker build cache (faster runner)

Stop wiping the layer cache + base images after every build (the host has 86GB free).
Keep base images (prune dangling only) and ~2 weeks of build cache, so the cache-friendly
Dockerfiles (COPY *.csproj + restore before source) actually benefit -> warm rebuilds skip
restore and base-image pulls.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -105,8 +105,8 @@ jobs:
         run: |
           docker push "${REGISTRY_HOST}/${PAGE_FETCHER_API_IMAGE}:${IMAGE_TAG}"
 
-      - name: Reclaim disk space
+      - name: Reclaim disk space (keep recent build cache)
         if: always()
         run: |
-          docker image prune -af
-          docker builder prune -af
+          docker image prune -f   # dangling only (keep base images)
+          docker builder prune -f --filter until=336h   # keep ~2 weeks of layer cache