Provide a way to add a tag to an existing image

[jsoref]

As described in #171, I have a multiarch image and want to add a new tag to it.

I'd really love to be able to just use manifest-tool tag <source-tag> <dest-tag> and have it do what I want.

I have a jsoref@retag-image script as a proof of concept.

-- It doesn't technically do the right thing for a single image, as it creates a manifest pointing to a single image instead of just adding a tag to the image, but I'm not sure there's a better way, and it saves me from downloading a 500+mb image when I don't need the image for my task. -- While the distinction in object types is technically observable, I suspect it doesn't practically matter.

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A manifest-tool copy <source-tag> <dest-tag> in the case where I need to cross repositories would also be great. (Although, I could understand if manifest-tool thinks that's outside its scope.)

[tianon]

You might appreciate regclient or crane 👀

(https://github.com/regclient/regclient, https://github.com/google/go-containerregistry/blob/main/cmd/crane/doc/crane.md)

[jsoref]

I've used crane for other tasks... (It's on my todo list). Adding regclient.

Indeed, it seems that I don't need manifest-tool here, although it would be nice if the front page had refs. As finding the right tool for these tasks is way more painful/frustrating than it should be...