Currently, `image.GetDiffID` cannot calculate DiffID of zstd layers because it
directly uses `compress/gzip` decompressor.
This commit fixes this issue by using the generic decompressor.
Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Go example:
```go
opts := []converter.Opt{
// convert Docker media types to OCI ones
converter.WithDocker2OCI(true),
// convert tar.gz layers to uncompressed tar layers
converter.WithLayerConvertFunc(uncompress.LayerConvertFunc),
}
srcRef := "example.com/foo:orig"
dstRef := "example.com/foo:converted"
dstImg, err = converter.Convert(ctx, client, dstRef, srcRef, opts...)
fmt.Println(dstImg.Target)
```
ctr example: `ctr images convert --oci --uncompress example.com/foo:orig example.com/foo:converted`
Go test: `go test -exec sudo -test.root -test.run TestConvert`
The implementation is from https://github.com/containerd/stargz-snapshotter/pull/224,
but eStargz-specific functions are not included in this PR.
eStargz converter can be specified by importing `estargz` package and using `WithLayerConvertFunc(estargz.LayerConvertFunc)` option.
This converter interface will be potentially useful for converting zstd and ocicrypt layers as well.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
