Ensure container generation uniformly reports outputs to MSBuild

[baronfel]

Fixes dotnet/sdk-container-builds#622.

Adds Returns to the single- and multi-arch container Targets so that regardless of how many containers are created consumers can get the whole list.

Adds an output of the generated container base image digest Label to the CreateNewImage Task so that the ContainerLabel itemgroup is updated after the Task is completed.

PR Overview

This pull request updates container generation logic so that both single- and multi-architecture builds uniformly report outputs to MSBuild while also providing the base image digest label for consumers. Key changes include:

• Adding a new [Output] property (GeneratedDigestLabel) in the CreateNewImage interface.
• Modifying the ImageBuilder to return a tuple for the base image digest label.
• Updating CreateNewImage to capture and assign the digest label output.

Reviewed Changes

File	Description
src/Containers/Microsoft.NET.Build.Containers/Tasks/CreateNewImage.Interface.cs	Added the new GeneratedDigestLabel output property.
src/Containers/Microsoft.NET.Build.Containers/ImageBuilder.cs	Modified AddBaseImageDigestLabel to return a tuple with the base image digest label.
src/Containers/Microsoft.NET.Build.Containers/Tasks/CreateNewImage.cs	Updated ExecuteAsync to destructure the tuple and assign the digest label output.

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/Containers/Microsoft.NET.Build.Containers/ImageBuilder.cs:110

• [nitpick] Consider using more descriptive tuple element names (e.g., 'digestLabelName' and 'digestValue') to clarify their purpose in the returned tuple.

internal (string name, string value) AddBaseImageDigestLabel()

src/Containers/Microsoft.NET.Build.Containers/Tasks/CreateNewImage.cs:164

• [nitpick] Consider renaming the tuple variables 'baseImageLabel' and 'baseImageDigest' to more descriptive names (e.g., 'digestLabelName' and 'digestValue') to better reflect their roles.

(baseImageLabel, baseImageDigest) = imageBuilder.AddBaseImageDigestLabel();

[baronfel]

@dotnet/product-construction there's a test failure in the ASP.NET tests in the sourcebuild leg that I think is unrelated to this change - it's a port-in-use failure.

[surayya-MS]

Looks good to me!