Dynamic plugin should support pulling image with digest

[susanshi]

What happened in your environment?

This issue was reported by a Ratify contributor:

Plugins with source property as a tag works, the same image with digest specified should also work.

Tag: wabbitnetworks.azurecr.io/test/sample-verifier-plugin:v1
digest: wabbitnetworks.azurecr.io/test/sample-verifier-plugin@sha256:294c517b9432a59f2d829f6d04912833a256560e4128aca86efa0c6b4ba6d44d

What did you expect to happen?

Current pulls with a digest fails with an error. Digest should be supported as tag is mutable.

What version of Kubernetes are you running?

No response

What version of Ratify are you running?

v1.1

Anything else you would like to add?

No response

Are you willing to submit PRs to contribute to this bug fix?

• Yes, I am willing to implement it.