Jackett works as a proxy server: it translates queries from apps (Sonarr, Radarr, SickRage, CouchPotato, Mylar3, Lidarr, DuckieTV, qBittorrent, Nefarious etc.) into tracker-site-specific http queries, parses the html or json response, and then sends results back to the requesting software.
$ mkdir -p data/{etc,var}
$ chown -R 1000:1000 data
$ docker-compose up -d
$ curl http://127.0.0.1:9117