What is the problem you're trying to solve

Motivation

Currently, nerdctl commit always creates images using Docker Schema2 media types (application/vnd.docker.container.image.v1+json, application/vnd.docker.distribution.manifest.v2+json). While this provides excellent Docker compatibility, it creates several critical issues:

Problem 1: Mixed Media Type Images

When committing containers that were created from OCI-format images, the current implementation creates mixed-format images with inconsistent media types:

{
    "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
    "schemaVersion": 2,
    "config": {
        "mediaType": "application/vnd.docker.container.image.v1+json",
        "digest": "sha256:1228f0866ba907da59e933fb318796968b4de520227a3b3875b94a94cb2bd3ec",
        "size": 7763
    },
    "layers": [
        {
            "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
            "digest": "sha256:9a8ece1256563321005c05a5e56e74f9b6b5033fd3b278156d5af768f2343b72",
            "size": 31413787
        },
        // ... more OCI layers from base image
        {
            "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
            "digest": "sha256:a483e3847f6fe662c0ae09609ed60ffad7902206bc0809b19bda895899043ba9",
            "size": 155
        }
    ]
}

Issues with this approach:

• Manifest and config use Docker Schema2 media types
• Existing layers from the base image retain OCI media types
• New layer from commit uses Docker media type
• Result: Inconsistent, mixed-format image

Problem 2: Tool Compatibility Issues

Mixed-format images cause failures with various container tools:

# skopeo fails to inspect mixed-format images
$ skopeo inspect docker://registry.domain.local/library/app1:after-commit
FATA[0000] Error parsing manifest for image: unsupported docker v2s2 media type: "application/vnd.oci.image.layer.v1.tar+gzip"

Other tools may also fail or behave unexpectedly when encountering these hybrid images.

Problem 3: Lack of Format Control

Users working in OCI-native environments have no way to:

• Maintain OCI format consistency when committing OCI-based containers
• Choose their preferred image format for committed images
• Ensure compatibility with their toolchain and registry requirements

Describe the solution you'd like

Solution: Add an option to nerdctl commit command for media type selection

This enhancement would:

• Eliminate mixed-format images: Ensure consistent media types throughout the image
• Provide format control: Users can choose the appropriate format for their workflow
• Maintain tool compatibility: Avoid failures with tools like skopeo.
• Support OCI-native workflows: Enable full OCI format support
• Preserve backward compatibility: Docker format remains default

Proposed Solution

Add a --format flag to the nerdctl commit command with two supported values:

• docker (default): Use Docker Schema2 media types
• oci: Use OCI image format media types

Command Usage

# Default behavior (Docker Schema2 format)
nerdctl commit container-name new-image:tag

# Explicitly specify Docker format
nerdctl commit --format docker container-name new-image:tag

# Use OCI format
nerdctl commit --format oci container-name new-image:tag

Media Type Mapping

Additional context

No response