fix: set media type to OCI for image cache layer

The other components of the image cache image are using OCI media types,
setting the layer to OCI instead of the default Docker layer type is
more technically correct. This also works around a bug in skopeo that
refuses to recognize Docker layers inside OCI images.

Signed-off-by: Joakim Nohlgård <joakim@nohlgard.se>
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

Author: jnohlgard

pkg/imager/filemap/filemap.go

@@ -17,6 +17,7 @@ import (
 
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/tarball"
+	"github.com/google/go-containerregistry/pkg/v1/types"
 )
 
 // File is a path -> file content map representing a file system.
@@ -151,5 +152,5 @@ func Layer(filemap []File) (v1.Layer, error) {
 	// Return a new copy of the buffer each time it's opened.
 	return tarball.LayerFromOpener(func() (io.ReadCloser, error) {
 		return Build(filemap), nil
-	})
+	}, tarball.WithMediaType(types.OCILayer))
 }