Add --prefer-index flag forimagetools create on a single source

[rvoh-tismith]

• Fixes issue imagetools create with single source changes copied image from application/vnd.docker.distribution.manifest.v2+json to application/vnd.docker.distribution.manifest.list.v2+json #2481
• I updated the tests that were affected by this change to test the new logic, however there were some tests in the repo that seem to have already been failing before I made any changes, so the few tests that are failing shouldn't have anything to do with my changes
• As a response to comments below, the default behavior isn't being changed -- the new logic is now behind the flag --prefer-index=false
• As a response to the comments below, I removed the mediaType check that I originally just added to because I can't really see a reason for it at this point
• Updated docs

[tonistiigi]

While I tend to agree that this makes more sense as a default behavior, this is removing the capability to convert image manifest into manifest list. Should we add a new flag that keeps previous behavior?

[tonistiigi]

If that is the desired behavior then why is there a mediatype check at all, or at least why is ocispec.MediaTypeImageManifest missing?

[rvoh-tismith]

Ahhhhh great point! Yeah, it actually seems reasonable that there doesn't even need to be a check. Thinking through the logic, I guess I can't really think of when the logic would need to be different here. Seems like if there is one source and no annotations (and possibly if the --preserve-single-source flag is set to true, based on my response to your other comment), we should just return the original bytes. I almost feel like I'm missing something here though. It feels like this is here for a reason, but maybe it is just an oversight.

[rvoh-tismith]

While I tend to agree that this makes more sense as a default behavior, this is removing the capability to convert image manifest into manifest list. Should we add a new flag that keeps previous behavior?

@tonistiigi great point. I was too focused on my own use case, but yeah, I definitely agree that some people are probably using this explicitly hoping to convert to a manifest list, or at the very least they've probably built workflows that are expecting this behavior, so changing the default behavior is probably too risky.

So, 2 things:

1. I think a flag for this behavior is the right way to go. I'm not married to the name, but maybe something like --preserve-single-source, so docker buildx imagetools create --preserve-single-source=true -t foo/bar:newimage foo/bar:oldimage.
2. Like I mentioned above, it feels like the behavior here should somewhat align with the behavior of docker buildx build which only outputs a manifest list if you specify multiple platforms. What are your thoughts there? Do you think there needs to be some alignment, or maybe the same flag for build so that you can control this behavior in both places? (that doesn't necessarily have to be included in this pr)

Thoughts?

[tonistiigi]

Maybe smth. like --prefer-manifest-index --create-index etc. for the behavior of always creating the index result and without the flag, it is the original manifest on a single source.

[rvoh-tismith]

Maybe smth. like --prefer-manifest-index --create-index etc. for the behavior of always creating the index result and without the flag, it is the original manifest on a single source.

Nice. I'll make some changes and push them up.

[rvoh-tismith]

I split the difference and landed on --prefer-index lol. I also removed the mediaType checks.