kaniko: Image built with Kaniko claims to be OCI but in reality is not

Actual behavior

Coming from https://github.com/containers/buildah/issues/3668

I am using kaniko to build an image based on an OCI-image.

The base image has the following manifest: ( notice mediaType: application/vnd.oci.image.layer.v1.tar+gzip )

>>> skopeo inspect --raw docker://${BASE_IMAGE} | jq .

{
  "schemaVersion": 2,
  "config": {
    "mediaType": "application/vnd.oci.image.config.v1+json",
    "digest": "sha256:61679cc9cfe1e3c757bfe2ff01222e25a4e0349ff70739f7c982df4e9484d5a4",
    "size": 419
  },
  "layers": [
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
      "digest": "sha256:3bc51580eb8a78b645a88f9c89c0779be50944543b917c682af93035002c2d99",
      "size": 79650768
    }
  ]
}

If I use this image as a base for another image built with Kaniko, I get the following resulting image:

{
  "schemaVersion": 2,
  "mediaType": "",
  "config": {
    "mediaType": "application/vnd.oci.image.config.v1+json",
    "size": 906,
    "digest": "sha256:da250df73fc4c57e758739f562f7c5bf77703f0547951a254a6043719ccb35a6"
  },
  "layers": [
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
      "size": 79650768,
      "digest": "sha256:3bc51580eb8a78b645a88f9c89c0779be50944543b917c682af93035002c2d99"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 222,
      "digest": "sha256:8962e548f920af01274257418f3414570b5d0761524773a86df7835344e467f7"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 198,
      "digest": "sha256:2b36e6ccfa17b538ef83c8aff96ffe823966883f90b4517b35346b57cc642c46"
    }
  ]
}

Which claims to be application/vnd.oci.image.config.v1+json but indeed has Docker application/vnd.docker.image.rootfs.diff.tar.gzip layers.

This shows as an error when going to use the child image as a base image in podman build, which shows (again see issue https://github.com/containers/buildah/issues/3668):

Error: error creating build container: error preparing image configuration: error converting image 
"containers-storage:[overlay@/var/lib/containers/storage+/run/containers/storage:overlay.mountopt=nodev]@04ee292f7a5549e765c99205acc567738a09eb084409cd71f6600facd3743c51" 
from "application/vnd.oci.image.manifest.v1+json" to "application/vnd.docker.distribution.manifest.v2+json": 
Unknown media type during manifest conversion: "application/vnd.docker.image.rootfs.diff.tar.gzip"

Expected behavior

As @vrothberg suggests, the layers should be converted to OCI ones during build or when pushing to the registry.

To Reproduce

Steps to reproduce the behavior:

• Have a base image with "mediaType": "application/vnd.oci.image.config.v1+json",
• Use the base image to build another (multistage) image with Kaniko.

Additional Information

• Dockerfile

Unfortunately it is quite difficult to find a public image that has "mediaType": "application/vnd.oci.image.config.v1+json", but to build one the following can be achieved with podman:

>> cat Containerfile
FROM docker.io/alpine
RUN touch file.txt
RUN echo "hello world"

>> podman build -t base-image -f Containerfile .
>> podman push base-image path/to/remote/repo/base-image

Dockerfile for child image:

FROM ubuntu:20.04 as installer
ADD installer.sh .
RUN bash installer.sh
######################
FROM path/to/remote/repo/base-image
COPY --from=installer /opt/application /opt/application
RUN ln -s /opt/application/1.0.0 /opt/application/stable
CMD ["/bin/bash"]

• Build Context Please provide or clearly describe any files needed to build the Dockerfile (ADD/COPY commands)

>>> cat installer.sh
mkdir -p /opt/application/1.0.0
touch /opt/application/1.0.0/file.txt
touch /opt/application/1.0.0/file2.txt
touch /opt/application/1.0.0/file3.txt

• Kaniko Image (fully qualified with digest)

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing	- [ ]
Please check if the build works in docker but not in kaniko	- [X]
Please check if this error is seen when you use --cache flag	- [ ]
Please check if your dockerfile is a multistage dockerfile	- [X]