Saving an image via digested reference, ID or truncated ID doesn't store
the image reference in the archive. This also causes the save code to
not add the image's manifest to the index.json.
This commit explicitly adds the untagged manifests to the index.json if
no tagged manifests were added.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit d131f00)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

When saving an image treat `image@sha256:abcdef...` the same as
`abcdef...`, this makes it:

- Not export the digested tag as the image name
- Not try to export all tags from the image repository

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 5e13f54)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 2ef0b53)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>