Some registries seem to be non-conformant and return a not found error
when pulling by digest (which docker now does all the time).
To work around this, fallback when all of the following are true:
1. Image reference is a tag
2. Tag->digest resolution succeeds
3. Fetch by resolved digest fails with a "not found" error.
This is intentionally not caching the manifests to reduce complexity for
this edge case.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
495d623ae5