Support for Multi-Architecture Index Manifest

[hardikdr]

Proposed Changes

This PR extends the image building, inspecting, listing, and pushing workflows to support [OCI index manifests](https://github.com/opencontainers/image-spec/blob/main/image-index.md) for multi-architecture images.

🔧 Key Changes

• Build command enhancements:

  • --multi-arch: Builds a top-level index manifest referencing per-arch layered manifests (e.g., amd64, arm64). Respects all per-arch inputs specified via --<flag>-<arch> format.
  • If --multi-arch is not used, the --arch flag becomes mandatory to clarify the target platform for a single image.

• Push command enhancements:

  • --push-sub-manifests: Pushes the index manifest along with all referenced architecture-specific sub-manifests.
  • Default behavior remains unchanged if this flag is omitted.

🧪 Example Test Command

./ironcore-image build \
  --multi-arch \
  --tag ghcr.io/ironcore-dev/os-images/test-image:100.4 \
  --rootfs-file-amd64 testdata/amd64/rootfs.img \
  --squashfs-file-amd64 testdata/amd64/squashfs.img \
  --initramfs-file-amd64 testdata/amd64/initramfs.img \
  --kernel-file-amd64 testdata/amd64/kernel.img \
  --uki-file-amd64 testdata/amd64/uki.img \
  --iso-file-amd64 testdata/amd64/iso.img \
  --rootfs-file-arm64 testdata/arm64/rootfs.img \
  --squashfs-file-arm64 testdata/arm64/squashfs.img \
  --initramfs-file-arm64 testdata/arm64/initramfs.img \
  --kernel-file-arm64 testdata/arm64/kernel.img \
  --uki-file-arm64 testdata/arm64/uki.img \
  --iso-file-arm64 testdata/arm64/iso.img

Feedback on the flag structure and UX is very welcome!!

Fixes #

[hardikdr]

For a quick test:

oras manifest fetch ghcr.io/ironcore-dev/os-images/test-image:100.4 | jq

[afritzler]

How about also adding --multi-arch --config=/path/to/config.yaml

architectures:
  amd64:
    rootfs: /path/to/amd64.rootfs
    kernel: /path/to/amd64.kernel
    # ...
  arm64:
    rootfs: /path/to/arm64.rootfs
    kernel: /path/to/arm64.kernel
    # ...

[hardikdr]

How about also adding --multi-arch --config=/path/to/config.yaml

Sounds good! I've added a --config flag to support parsing from a file -- if it's not provided, it’ll just fall back to the command-line flags.