A simple (also minimal) torznab protocol implement which can work with sonarr.
Put http://your-ip:8000/ in settings and leave anything in api key.
docker pull htynkn/wkavu
docker run -d --name wkavu --privileged -v /mnt/appdata/wkavu:/data -e DB_URL=sqlite:///data/db.db -p 8991:8000 htynkn/wkavu
Docker image tested with TNAS F4-221