Just running:

docker run ubuntu yes "some long string"

Will make docker eat all the memory...

More observations:

1. Memory reclaimed only when container is removed
2. It might leak only when docker run is attached
3. It doesn't seem that docker run can't cope with the traffic (the leak discovered on real logging not on "yes" command)
4. No "attach" or "logs" command in the process. But "docker run" is attached.