This is the official discovery server image, which is what runs on announce.syncthing.net.
To start,
docker run -p 22026:22026/udp --restart=always syncthing/discosrv &
The container will be automatically restarted on boot or if the
discovery server crashes for whatever reason. The database and
statistics are stored in a volume on /var/discosrv, which can be
inspected from another container if necessary.