fix(save): use Docker v2s2 manifest format for skopeo compatibility

Sources/tart/OCI/Manifest.swift

@@ -4,6 +4,10 @@ import Foundation
 let ociManifestMediaType = "application/vnd.oci.image.manifest.v1+json"
 let ociConfigMediaType = "application/vnd.oci.image.config.v1+json"
 
+// Docker manifest and config media types (schema v2)
+let dockerManifestMediaType = "application/vnd.docker.distribution.manifest.v2+json"
+let dockerConfigMediaType = "application/vnd.docker.container.image.v1+json"
+
 // Layer media types
 let configMediaType = "application/vnd.cirruslabs.tart.config.v1"
 let diskV2MediaType = "application/vnd.cirruslabs.tart.disk.v2"

Sources/tart/OCI/OCIArchiveWriter.swift

@@ -66,7 +66,7 @@ extension OCIArchiveWriter: BlobStorage {
     var manifests: [[String: Any]] = []
 
     let baseDescriptor: [String: Any] = [
-      "mediaType": ociManifestMediaType,
+      "mediaType": dockerManifestMediaType,
       "digest": manifestDigest,
       "size": manifestSize,
     ]

Sources/tart/VMDirectory+OCIArchive.swift

@@ -29,12 +29,17 @@ extension VMDirectory {
     let ociConfigContainer = OCIConfig.ConfigContainer(Labels: labels)
     let ociConfigJSON = try OCIConfig(architecture: config.arch, os: config.os, config: ociConfigContainer).toJSON()
     let ociConfigDigest = try await archive.pushBlob(fromData: ociConfigJSON, chunkSizeMb: 0, digest: nil)
-    let manifest = OCIManifest(
-      config: OCIManifestConfig(size: ociConfigJSON.count, digest: ociConfigDigest),
+
+    var manifestConfig = OCIManifestConfig(size: ociConfigJSON.count, digest: ociConfigDigest)
+    manifestConfig.mediaType = dockerConfigMediaType
+
+    var manifest = OCIManifest(
+      config: manifestConfig,
       layers: layers,
       uncompressedDiskSize: UInt64(diskSize),
       uploadDate: Date()
     )
+    manifest.mediaType = dockerManifestMediaType
 
     let tagRef = tag ?? "latest"
     defaultLogger.appendNewLine("saving manifest...")