fix: avoid unneccesary temporary layout during build --oci

With sylabs#2935 in place we can now translate a Docker archive tarball into
an OCI-SIF without creating a redundant copy of the image as a temporary
OCI layout.

We've previously been processing the buildkit tarball as an oci-archive,
which still requires extraction to a temporary layout, as ggcr's tarball
package doesn't accept an archived layout without a manifest.json.

Because the buildkit output tarball includes the Docker specific
manifest.json, in addition to the standard OCI layout files, it can be
treated as a docker-archive - now avoiding the need for the temporary
layout.

Significantly reduces disk space needed by `build --oci`, and the
associated I/O.

Fixes sylabs#2308

CHANGELOG.md

@@ -16,6 +16,8 @@
 - Avoid unnecessary copying / extraction of OCI images and Docker tarballs into
  a layout directory when they are directly accessible as a local file /
  directory.
+- Avoid unnecessary intermediate temporary image layout when building from
+ Dockerfile to OCI-SIF.
 
 ## 4.1.3 \[2024-05-08\]
 

internal/pkg/build/buildkit/client/client.go

@@ -123,7 +123,7 @@ func Run(ctx context.Context, opts *Opts, dest, spec string) error {
  }
  pullOpts.Platform = *platform
  }
- if _, err := ocisif.PullOCISIF(ctx, nil, dest, "oci-archive:"+tarFile.Name(), pullOpts); err != nil {
+ if _, err := ocisif.PullOCISIF(ctx, nil, dest, "docker-archive:"+tarFile.Name(), pullOpts); err != nil {
  return fmt.Errorf("while converting OCI tar image to OCI-SIF: %w", err)
  }