Check the docker image that is pulled has the correct id

[Ugo]

Currently if an app dev pushes a new version of a docker image with a same tag, the worker will not see the difference when executing an task based on an old version of the image. Since the id of the docker image that should be used in the task is available, it should be used to check that the worker executes the task on the right docker image.

[Ugo]

The repoDigest of a docker image can be obtained with the following command:
docker inspect --format='{{.RepoDigests}}' IMAGE_NAME

[jeremyjams]

I think we can merge this issue with that one : #91