feat(builder): Add rootfs implementation for reading OCI images

[craciunoiuc]

Tested work and it seemed fine. Pasting again uses:
Case 1:

rootfs:
  format: erofs
  type: oci
  source: index.unikraft.io/test/check-pr:latest

rootfs:
  format: erofs
  source: oci://index.unikraft.io/test/check-pr:latest

Case 2:

rootfs:
  format: erofs
  type: oci
  source: index.docker.io/hello-world:latest

rootfs:
  format: erofs
  source: oci://index.docker.io/hello-world:latest

You can use these prefixes: oci://, oci-layout://, oci-archive:// taken from image-spec:

case imagespec.URISchemeOCI, imagespec.URISchemeOCILayout, imagespec.URISchemeOCIArchive:

Closes: TOOL-1073

[craciunoiuc]

let me know if these single use functions you want to inline

I found them significant enough to keep separate for now

[Copilot]

Pull request overview

Adds support in the builder to treat OCI image references (registry, OCI layout, OCI archive) as rootfs sources, including logic to unpack regular OCI layers into a rootfs and to use a Unikraft-specific initrd component when present.

Changes:

• Detect OCI rootfs sources via URI schemes and route BuildRootfs to a new OCI implementation.
• Implement buildRootfsOCI to load OCI images per requested platform, flatten layers into a directory, and package into CPIO/EROFS.
• Update kraftfile-to-buildopts translation and expand unit/integration-style tests for OCI sources.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/builder/rootfs.go	Adds OCI source detection + buildRootfsOCI (load, platform match, layer flatten, repack).
internal/builder/rootfs_test.go	Adds DetectSourceType tests for OCI schemes and integration-style rootfs OCI tests.
internal/builder/rootfs_oci_test.go	Adds helpers to generate local OCI archives used by tests.
internal/builder/kraftfile.go	Avoids filepath-joining when rootfs/ROM source is an OCI reference or explicitly typed OCI.
internal/builder/kraftfile_test.go	Adds coverage for OCI paths/types in kraftfile-to-buildopts conversion.
go.mod	Pulls in new (indirect) deps required by containerd/v2 usage.
go.sum	Updates sums for newly introduced transitive dependencies.

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[craciunoiuc]

@jedevc sounds 🧢

should I change?