[FEATURE] Include docker image digest to mlcube id for `medperf test`

[VukW]

During mlcube development user builds & runs image with the same tag a lot of times, after every change. Right now only mlcube/ folder hash is used for calculating temporary mlcube id; it includes image tag, but not image digest. It means, that if the image is updated & rebuilt, mlcube/ folder is not changed => model id is not changed => medperf behaves as it is running exactly the same model (in particular, not recalculating predictions). That goes silently and is not so obvious to the user.

I propose to add image digest during temporary mlcube id calculation, so updated image would be treated as a totally new one.