Prune image API not working

[e01953]

## Expected/Wanted Behavior
All unused images should get prune

## Actual Behavior
Pruning not working

## Steps to Reproduce the Problem

1. Create an app (e.g. 'nativeexecution') with hello-world as docker image and associate with device group(e.g.iotDevice)
2. Update the docker image as "nginx" in app(nativeexecution) with configuration API.
3. docker images in terminal -> 3 images are showing 1) Nebula worker 2) hello-world 3) nginx
4. Used the prune API to remove the unused Images but all images are showing none of them was prune.

[issue-label-bot]

Issue-Label Bot is automatically applying the label bug to this issue, with a confidence of 0.93. Please mark this comment with 👍 or 👎 to give our bot feedback!

Links: app homepage, dashboard and code for this bot.

[naorlivne]

We seems to have hit docker/docker-py#1939 with this bug, the fix stated in the link should work here as well so I'll add that in

[naorlivne]

pushed to latest, fix will be added to next versioned build (but you can of course test on latest meanwhile)