Conversion to schema1 does not fail with Zstd layers, making it uncertain we correctly convert to OCI

[mtrmac]

With Podman pushing a zstd:chunked image:

DEBU[0000] Manifest has MIME type application/vnd.docker.distribution.manifest.v2+json, ordered candidate list [application/vnd.docker.distribution.manifest.v2+json, application/vnd.docker.distribution.manifest.v1+prettyjws, application/vnd.oci.image.manifest.v1+json, application/vnd.oci.image.index.v1+json, application/vnd.docker.distribution.manifest.list.v2+json, application/vnd.docker.distribution.manifest.v1+json] 
DEBU[0000] ... will first try using the original manifest unmodified 
…
DEBU[0000] Writing manifest using preferred type application/vnd.docker.distribution.manifest.v2+json failed: creating an updated image manifest: preparing updated manifest, layer "sha256:208a74e284fc3d9c8397655bd66f5b927d94354a7fe5fcdc084df4aa792c61fe": zstd compression is not supported for type "application/vnd.docker.image.rootfs.diff.tar.gzip" 
DEBU[0000] Trying to use manifest type application/vnd.docker.distribution.manifest.v1+prettyjws… 

^^

DEBU[0000] exporting opaque data as blob "sha256:a416a98b71e224a31ee99cff8e16063554498227d2b696152a9c3e0aa65e5824" 
DEBU[0000] Uploading empty layer during conversion to schema 1 
DEBU[0000] Checking /v2/my-busybox/blobs/sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4 
DEBU[0000] HEAD http://localhost:5000/v2/my-busybox/blobs/sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4 
DEBU[0000] ... already exists                           
Writing manifest to image destination
DEBU[0000] PUT http://localhost:5000/v2/my-busybox/manifests/latest 

The conversion has apparently succeeded, and we would be happy with a schema1 upload!

DEBU[0000] Error uploading manifest latest to localhost:5000/my-busybox: received unexpected HTTP status: 500 Internal Server Error while writing manifest "…" 
DEBU[0000] Upload of manifest type application/vnd.docker.distribution.manifest.v1+prettyjws failed: writing manifest: uploading manifest latest to localhost:5000/my-busybox: received unexpected HTTP status: 500 Internal Server Error 

We just got lucky, the registry doesn’t support schema1

DEBU[0000] Trying to use manifest type application/vnd.oci.image.manifest.v1+json… 

… and finally trying to do the right thing.

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At the very least, UpdatedImage must refuse to convert to schema1+zstd, or to accept re-compression to zstd. (Probably primarily Schema1.UpdateLayersInfos.)

The copy.determineManifestConversion code (and the multi-platform analogue?) might also want to be aware of compression algorithms, so that we only try OCI and don’t bother with the schema2/schema1 conversions.

[mtrmac]

The copy.determineManifestConversion code (and the multi-platform analogue?) might also want to be aware of compression algorithms, so that we only try OCI and don’t bother with the schema2/schema1 conversions.

That’s actually required: #2195.