Add support for Labels to the Image config

[timheuer]

Adds support for ContainerLabel item group properties.

<ItemGroup>
  <ContainerLabel Include="com.microsoft.created-by" Value="dotnet-sdk" />
  <ContainerLabel Include="com.microsoft.project-name" Value="$(MSBuildProjectName)" />
</ItemGroup>

[baronfel]

This is good to go now. I added an initial conventional label to prove that the mechanism works. On my local testing, we go from:

"Labels": null

to this:

"Labels": {
    "org.opencontainers.image.created": "2022-08-19T15:03:03.7298106Z"
}

[baronfel]

Thanks for this!

[baronfel]

Fixes #95