docker-archive: multiple tag support

[cyphar]

At the moment if you want to operate on a docker-archive: it will only create a single RepoTags entry. However, Docker image tarballs are quite different to other images in that when you import them into docker you have no way of changing their name -- so if you wanted to import a single image under multiple names you would have to skopeo copy the image to different archives. However the format of docker load supports more than one entry in RepoTags.

The idea would be to add the ability to have "alternative names" in SystemContext (or something similar) for the ImageDestination. This would result in having multiple tags created referencing the image we've decided as our destination. Then we could plumb through an option like --dst-additional-tags to skopeo. Ultimately this could be added to all transports, though I'm not sure whether it would work very well with all of them.

WDYT?

/cc @vrothberg

[mtrmac]

Sure. The feature makes sense; exposing it through SystemContext seems to be the best of the various ugly options.

Maybe make the SystemContext field / skopeo option explicitly docker-archive:-specific, at least until we have more enough other transports to understand the common/different aspects. (E.g. oci-archive: might in principle benefit from something similar, but uses a different reference syntax.)

[mtrmac]

Looking at containers/podman#649 , do you need to handle only the case of multiple tags for a single image, or do you expect to need support for true multi-image archives? The latter would make adding special-case SystemContext facilities somewhat less attractive (though still acceptable to me).

Cc: @runcom

[vrothberg]

Just to clarify: this issue and containers/podman#649 are only indirectly related. The current issue mostly relates to skopeo-copy dealing with multiple tags. In this context, we checked how podman deals with it and found the issues mentioned in containers/podman#649.

[...] do you need to handle only the case of multiple tags for a single image, or do you expect to need support for true multi-image archives?

We only need multiple tags for a single image.

[vrothberg]

Here's a PR implementing it: #450

I used this commit in skopeo copy to add additional tags via a --additional-tag flag, and it works as expected (i.e., the manifest.json in the archive includes the additional tags).