`convox start` vs `docker-compose up` stdout interleaving

gistfile1.txt

`docker-compose up` of a simple `echo hello world` container gives me

Recreating dockerrig_sonar_1
Attaching to dockerrig_sonar_1
sonar_1 | hello world

whereas `convox start` of the same environment gives me

RUNNING: docker build -t vdlpyobjtp /Users/paul/src/wipro/docker-rig/sonar
Sending build context to Docker daemon 4.096 kB
Step 1 : FROM centos
 ---> 61b442687d68
[...blah...]
 ---> 5236075ada9f
Successfully built 5236075ada9f
RUNNING: docker tag -f vdlpyobjtp docker-rig/sonar
Warning: '-f' is deprecated, it will be removed soon. See usage.
sonar | running: docker run -i --name docker-rig-sonar docker-rig/sonar

Note the missing expected "sonar | hello world", as would be implied by https://convox.com/docs/developing-locally/

docker-compose version
docker-compose version 1.6.0, build d99cad6
docker-py version: 1.7.0
CPython version: 2.7.9
OpenSSL version: OpenSSL 1.0.1j 15 Oct 2014

convox --version
client: 20160128232541
server: 20160129191924