Support pulling an OCI artifact/image from registry to a local OCI archive

[FeynmanZhou]

What is the version of your ORAS CLI

v1.1.0

What would you like to be added?

We have seen multiple ORAS users who wanted to pull OCI artifact or container images from a remote registry to a local OCI archive or an OCI layout. oras copy has options to pull the subject/referrer from remote registry to OCI layout (or from OCI layout to registry).

However, Pull is commonly understood but Copy is an extra cognitive step for most of the new ORAS users. We need to design and provide a way to enable users easily pull an OCI artifact/image from registry to a local OCI archive or an OCI layout.

Why is this needed for ORAS?

In order to provide a natural experience of pulling content from registry to local and make the design principle conforms to users’ default habits.

We received several ORAS user requests on this feature. Related discussion: https://cloud-native.slack.com/archives/CJ1KHJM5Z/p1697701836678159?thread_ts=1697645768.389369&cid=CJ1KHJM5Z

Are you willing to submit PRs to contribute to this feature?

• Yes, I am willing to implement it.

[shizhMSFT]

In terms of local OCI archive, does it mean a .tar tarball, a gzip one .tar.gz, or simply a .zip file?

What if there is already an existing OCI archive? For example pulling a bar:latest image to foo.tar, which contains foo:latest. Will it overwrite or store both foo:latest and bar:latest in a single foo.tar file?

[shizhMSFT]

Since oras pull is about pulling extracted artifact files, do we need a new command like oras export or oras save or oras dump for pulling an image from registry to a local archive?

[shizhMSFT]

Or consider something like oras pull --output-oci-layout <file_path>.

[fsdrw08]

maybe oras save, just like what docker/podman does, but also need to add --format flag like podman to save the image as oci-archive

[shizhMSFT]

Linking

• Add new commands to support uploading and downloading images #1366