oci-fetch: Fix wrong oci layout

[sgotti]

oci fetch is generating a wrong oci layout (doesn't pass oci-image-tool validation) because oci-fetch was writing inside the refs/ dir the manifest instead of the descriptor.

This patch fixes this doing multiple things:

• write a descriptor inside the refs/ dir
• save the manifest as a blob inside blobs/ dir
• keep the retrieved manifest data (without unmarshall/marshall) since this will change the file digests.
• remove leading / in tar entries

This has been tested by temporary uncommenting the docker v2.2 mediaTypes in lib/schema/mediatype.go

[runcom]

yet, we have almost the same stuff in skopeo (working exactly the same as this repo+this commit does) and this sounds a really waste of resources :/ containers/image will soon have the ability to do what this repo does with just like 2 function calls, I know Simone is working on his OCI store with a custom client.... would be awesome to move everything to containers/image..

so me, Simone and Derek did the same thing 3 times and we fix each other bug :)

This binary here will eventually directly use containers/image to retrieve an OCI image...instead of replicating the whole logic of saving to an image-layout...

[sgotti]

@runcom I agree. I'll go for https://github.com/containers/image and suggest to use skopeo or just let oci-fetch become a https://github.com/containers/image example usage.

@dgonyeo @philips If you prefer let's discuss it in https://groups.google.com/forum/?hl=en#!forum/containers-dev google group

[runcom]

I also think this tool isn't going to work if Docker Hub returns version 2 schema 1 manifests because they're not compatible with OCI v1 manifest

[cgonyeo]

@runcom that's correct, but I don't know how an OCI-only fetching tool would handle that.

[runcom]

It would not :)