logspout-logstash is based on the Loop Lab repository (https://github.com/looplab/logspout-logstash). This is hosted publicly so that others may simply run it as needed. It maintains the syslog rfc5424 format.
You can run this container with
docker run -d --name logspout-logstash --restart always -e ROUTE_URIS=logstash://[your_logstash_host_and_port] -v /var/run/docker.sock:/tmp/docker.sock jmstone617/logspout-logstash