Set mediaType from http response if manifest mediaType is not present

[surayya-MS]

Context

Fixes dotnet/sdk-container-builds#567.

Customer impact

Some (rare) images do not have a media type, which instructs container tooling how to interpret the data in the container image. Our tooling incorrectly interpreted the spec and required the mediaType to be present, instead of probing and defaulting to a fallback mediaType if one is present. Without this change, users using custom base images may be blocked from using the SDK tooling.

Details

Manifest MediaType field is optional in the docs.

I found this when I was trying to set base image as an oci image. For that i used skopeo to convert aspnet 8 to oci image.

skopeo copy docker://mcr.microsoft.com/dotnet/aspnet:8.0 oci:/path/to/aspnet_oci:latest
skopeo oci:/path/to/aspnet_oci:latest docker://docker.io/{user}/my-aspnet:8.0

that image manifest didn't have mediaType. I got "error MSB4018: The "CreateNewImage" task failed unexpectedly." that failed here :

sdk/src/Containers/Microsoft.NET.Build.Containers/Registry/Registry.cs

Line 213 in 1ed097e

var manifest = (await initialManifestResponse.Content.ReadFromJsonAsync<ManifestV2>(cancellationToken: cancellationToken).ConfigureAwait(false))!;

Changes

Made ManifestV2.MediaType optional
Set mediaType from http response content for image building in case when manifest mediaType is not present.

Related #40776

Risk

Low - These images are rare in the wild, though some fairly-widely-used tooling does seem to trigger this corner case.

[baronfel]

Approved over email.

[baronfel]

Merging as the branch versions are bumped!