Modified to passthrough log messages already in JSON format and add the docker specific fields added. Instead of creating a new JSON object and included the JSON-log message as escaped JSON in the 'message' field.
A minimalistic adapter for github.com/gliderlabs/logspout to write to Logstash TCP
Follow the instructions in https://github.com/gliderlabs/logspout/tree/master/custom on how to build your own Logspout container with custom modules. Basically just copy the contents of the custom folder and include github.com/looplab/logspout-logstash in modules.go.
Use by setting ROUTE_URIS=logstash://host:port to the Logstash host and port for TCP.
In your logstash config, set the input codec to json e.g:
input { tcp { port => 5000 codec => json } }