Track time of release

[AlexGustafsson]

We could get the release time of a version by looking at the manifest, which contains "created" and "published" timestamps. These could be used to improve the UI and RSS feed with accurate information of when a release was made. This could also be helpful for things like "latest" images where the tag remains the same, but the actual service can differ a lot over time.

This is probably best done in the common OCI part of the workflow, by getting the manifest for the latest reference if it exists. The current manifest is already being fetched.

[AlexGustafsson]

Note that when finding a "latest" image in Docker or Kubernetes, it would be helpful to know the digest. That way we can diff the current "latest" vs the latest available "latest". Solely getting the "latest" manifest will not allow us to show the user that the image is actually outdated.

The same is true for some tags such as "v2" where the tag can be pushed to every now and then. Differentiating between versions of the same tag would be rather powerful.