When pull image via digest is supported ( #1186 ) we need to ensure that the digest format is sent to the client correctly.

Currently the function to format the digest for the client will return the following formatted digest:

repo@sha:digest (i.e. busybox@sha:1235453303)

If should instead return:

repo@sha256:digest (i.e. busybox@sha256:121212121213)

A simple change, but one that must be made so that docker images --digests is supported.