Allow synchronization with sha syntax

[nudgegoonies]

For images that only have a latest tag it would be useful to sync an image via SHA. This is an example image that we stumbled on where we mirror the SHA manually:

solsson/kafka-prometheus-jmx-exporter@sha256:6f82e2b0464f50da8104acd7363fb9b995001ddff77d248379f8788e78946143

[ivanilves]

Good idea @nudgegoonies 👍 Will look into it.