Correct host image cleanup: prune dangling only, keep tagged :latest

The previous cleanup guidance removed the tagged :latest after push,
which deleted images still deployed on the Unraid host. Replace with
a dangling-only prune in both the troubleshooting doc and the sample
docker-build-push workflow; validated on jason/wfh (reclaimed 4.3GB).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/sample-workflows/docker-build-push.yml

@@ -36,3 +36,11 @@ jobs:
             .
           docker push "${IMAGE}:latest"
           docker push "${IMAGE}:${{ gitea.sha }}"
+
+      # Dangling-only prune: removes untagged leftovers from previous builds.
+      # Never removes tagged images (the fresh :latest may be deployed on the
+      # host) or any image referenced by a container. See
+      # docs/troubleshooting.md "Orphan images piling up on the host".
+      - name: Prune dangling images on host
+        if: always()
+        run: docker image prune -f 2>/dev/null || true