question: support for v1layout tarball?

[deitch]

The v1/tarball.Write() writes it in docker's unique tarball format. Per the summary:

This package produces tarballs that can consumed via docker load. Note that this is a different format from the legacy tarballs that are produced by docker save, but this package is still able to read the legacy tarballs produced by docker save.

As of docker engine v25, docker save actually writes to OCI layout tarball, i.e.:

$ docker save alpine:3.19 > /tmp/alpine319.tar
$ tar xvf /tmp/alpine319.tar
x blobs/
x blobs/sha256/
x blobs/sha256/a0264d60f80df12bc1e6dd98bae6c43debe6667c0ba482711f0d806493467a46
x blobs/sha256/ace17d5d883e9ea5a21138d0608d60aa2376c68f616c55b0b7e73fba6d8556a3
x blobs/sha256/bca4290a96390d7a6fc6f2f9929370d06f8dfcacba591c76e3d5c5044e7f420c
x blobs/sha256/c5b1261d6d3e43071626931fc004f70149baeba2c8ec672bd4f27761f8e1ad6b
x index.json
x manifest.json
x oci-layout

That is almost identical to v1 layout, which GGCR supports, with the exception of:

• it is in a tar stream, not an actual layout
• it includes docker's unique manifest.json

FWIW, this is identical to the results of using buildkit with --output type=oci,path=/tmp/foo.tar

Is there a way to output this tar format? In theory, we could output to a directory using layout.Write() and then tar it up, but that is inefficient and could get messed up, and in any case, it all is a tar stream.