Remove dependency on docker_content_digest digest headers for docker updates

[yeikel]

Code improvement description

Depending on this header to get around rate limits is not desired because this header is not present in every Docker Registry Implementation:

Because of the origins this specification, the client MAY encounter Docker-specific headers, such as Docker-Content-Digest, or Docker-Distribution-API-Version. These headers are OPTIONAL and clients SHOULD NOT depend on them.

In my opinion, what we should do instead is to retry

Additional Context :

deitch/docker_registry2#98

#7578 (comment)