Include container image short hash in actor record

[mwvaughn]

I propose including the short hash for the deployed container in the /actors response to help validate that the correct image is deployed. In theory the org/image:tag|hash should be good enough but a common use finds container with semantic versioning schemes and so may be rebuilt several times while iterating on functionality.

Current

{   "id": "w7LMK0k7JGZZQ",
    "image": "sd2e/agave-test:dev",
    "lastUpdateTime": "2018-04-19 16:32:00.506656" }

Proposed

{    "id": "w7LMK0k7JGZZQ",
    "image": "sd2e/agave-test:dev",
    "image_id": "8e481d5e3679",
    "lastUpdateTime": "2018-04-19 16:32:00.506656"  }

where image_id is the value from docker images sd2e/agave-test:dev:

docker images sd2e/agave-test:dev
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
sd2e/agave-test     dev                 8e481d5e3679        11 minutes ago      629MB