Manifests are now fetched by a field called "reference", which may be a tag or
a digest. When using digests to reference a manifest, the data is immutable.
The routes and specification have been updated to allow this.
There are a few caveats to this approach:
1. It may be problematic to rely on data format to differentiate between a tag
and a digest. Currently, they are disjoint but there may modifications on
either side that break this guarantee.
2. The caching characteristics of returned content are very different for
digest versus tag-based references. Digest urls can be cached forever while tag
urls cannot.
Both of these are minimal caveats that we can live with in the future.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
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