refactoring: make BuiltImage a class instead of a struct

Author: Surayya Huseyn Zada

src/Containers/Microsoft.NET.Build.Containers/BuiltImage.cs

@@ -6,7 +6,7 @@ namespace Microsoft.NET.Build.Containers;
 /// <summary>
 /// Represents constructed image ready for further processing.
 /// </summary>
-internal readonly struct BuiltImage
+internal sealed class BuiltImage
 {
     /// <summary>
     /// Gets image configuration in JSON format.
@@ -46,12 +46,12 @@ internal readonly struct BuiltImage
     /// <summary>
     /// Gets image OS.
     /// </summary>
-    internal string OS { get; init; }
+    internal string? OS { get; init; }
 
     /// <summary>
     /// Gets image architecture.
     /// </summary>
-    internal string Architecture { get; init; }
+    internal string? Architecture { get; init; }
 
     /// <summary>
     /// Gets layers descriptors.

src/Containers/Microsoft.NET.Build.Containers/ImageIndexGenerator.cs

@@ -71,8 +71,8 @@ private static (string, string) GenerateImageIndex(BuiltImage[] images, string m
                 digest = image.ManifestDigest,
                 platform = new PlatformInformation
                 {
-                    architecture = image.Architecture,
-                    os = image.OS
+                    architecture = image.Architecture!,
+                    os = image.OS!
                 }
             };
             manifests[i] = manifest;

src/Containers/Microsoft.NET.Build.Containers/LocalDaemons/DockerCli.cs

@@ -510,7 +510,7 @@ private static async Task WriteIndexJsonForMultiArchOciImage(
                 mediaType = SchemaTypes.OciManifestV1,
                 size = images[i].Manifest.Length,
                 digest = images[i].ManifestDigest,
-                platform = new PlatformInformation { architecture = images[i].Architecture, os = images[i].OS }
+                platform = new PlatformInformation { architecture = images[i].Architecture!, os = images[i].OS! }
             };
             manifests[i] = manifest;
         }