Investigate what to have as image build output (jib-image.json, jib-image.digest, and jib-image.id) for multi-platform image building

[chanseokoh]

Should we write multiple jib-image.digest and jib-image.id files for each manifest? What about the file names? Should we also write a digest of a manifest list (potentially with a different name)? Or, should we write only a single digest for a manifest list?

Does it make sense change the JSON format of jib-image.json? Should we keep backward compatibility?

On the technical side, we need to figure out the right return type of PushImageStep after sorting out these questions:

jib/jib-core/src/main/java/com/google/cloud/tools/jib/builder/steps/PushImageStep.java

Lines 40 to 46 in 94f8bda

	/**
	* Pushes a manifest or a manifest list for a tag. If not a manifest list, returns the manifest
	* digest ("image digest") and the container configuration digest ("image id") as {@link
	* BuildResult}. If a manifest list, returns the manifest list digest only.
	*/
	// TODO: figure out the right return value and type when pushing a manifest list.
	class PushImageStep implements Callable<BuildResult> {

[tom-haines]

@chanseokoh if relevant, whilst docker exporter does not currently support exporting manifest lists it does allow to output tar bundles, but only supports one platform at a time.

Example buildx commands re amd64 first, and arm64 at end:

docker buildx build --platform linux/amd64 -o type=docker,dest=/tmp/helloworld/amd64.tar .
tar xf -C amd64 amd64.tar
tree .

output:

├── Dockerfile
├── amd64
│   ├── blobs
│   │   └── sha256
│   │       ├── 16ec32c2132b43494832a05f2b02f7a822479f8250c173d0ab27b3de78b2f058
│   │       ├── 1b307460a9df24f41e954a977e17a389633f6cde89fd75efa93b4259a69f8966
│   │       ├── 34f1332d72bf8b6edcc1a4d11f3310cb84c0a69b6f9374dad77253ad07c4f308
│   │       └── f90645af5f5b704bdd16861777f8ac9c50a405a397b7cb32d6cf34b6f4b928ee
│   ├── index.json
│   ├── manifest.json
│   └── oci-layout
└── amd64.tar

$ cat amd64/manifest.json | jq -r

output:

[
  {
    "Config": "blobs/sha256/34f1332d72bf8b6edcc1a4d11f3310cb84c0a69b6f9374dad77253ad07c4f308",
    "RepoTags": null,
    "Layers": [
      "blobs/sha256/16ec32c2132b43494832a05f2b02f7a822479f8250c173d0ab27b3de78b2f058",
      "blobs/sha256/f90645af5f5b704bdd16861777f8ac9c50a405a397b7cb32d6cf34b6f4b928ee"
    ]
  }
]

any platforms available via docker buildx build can be saved locally.

example

docker buildx build --platform linux/arm64 -o type=docker,dest=/tmp/helloworld/arm64.tar .

It seems to support building each image via qemu, but omits creating a mutli-platform manifest wrapper.

FROM ubuntu:20.04
RUN apt-get update && apt-get install -y curl

[chanseokoh]

@tom-haines thanks for the info!