Description of problem:

Tags are not deterministic. They are mutable and volatile. There is no guarantee that in time same content will be pointing to tag e.g. 'latest'.

Describe the solution you'd like:
To solve the issue of reproducibility one could be using digests, as they are content addressable and instead mirror content by digest.
Container registry does not provide API endpoint that would list all manifest digests present within the repo as it is possible with tags/list endpoint, hence we need to add digests_list field to pulp-container-remote that will enable user to provide a list of manifest digest to mirror from the remote registry.

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