docker exec may report 'mnt_dir missing value' error/warning

[grahamwhaley]

System:
(ciao-down clearcontainers: https://github.com/grahamwhaley/ciao/tree/master/testutil/ciao-down)
Ubuntu 16.04.3 LTS
Docker 1.12.1
cc-oci-runtime 2.2.4

What did I do:

docker run -d --name=fred busybox sh -c 'tail -f /dev/null'`
docker exec fred sh -c 'pwd'

What did I see:

{"level":"critical","mesg":"mnt_dir missing value","time":"2017-08-18T15:16:08.623891Z"}
/

What I didn't expect to see is the mnt_dir missing value error message.
I've not managed to track down where this comes from, but I suspect docker, and @jodh-intel suspects something in the mount parts of the config.json.

For reference:
I found this whilst running the CC3.x metrics on CC2.x (specifically https://github.com/clearcontainers/tests/blob/master/metrics/network/network-metrics-memory-pss-1g.sh)
I do not see this when running CC3.0.0-beta1 under Ubuntu bare metal 16.04 and docker 17.03-ce.