docker.tarfile: support multiple images in the same archive

[cyphar]

From a source perspective, this would be implemented as selecting what RepoTag to extract layers from. And from a destination perspective, it would be implemented by changing what the resultant RepoTags already present at a destination are.

Essentially all of this would be controlled by choosing a full reference when creating references that use docker.tarfile.

[rhatdan]

@cyphar @mtrmac @vrothberg What is the state of this issue. It is two years old, can we close it?

[vrothberg]

The issue remains but I think we can close it given the age of the issue.

[mtrmac]

Age or not, this was actually requested recently — but in a duplicate which has more details on a possible implementation approach, so, sure, we can close this one.

To link the two, #610 .