Support forcing OCI layout exports even when base images are Docker formats

[baronfel]

Some export tools only support OCI formats these days, so we need a way for users to explicitly request OCI formats, regardless of the format of the base image. This should be a lossless conversion on our part, we just need a user signal exposed.

Proposal

<PropertyGroup>
  <!-- Valid values are OCI or Docker. Controls what the format of any images or tarballs will be.  
      If not specified, defaults to that of the base image used. -->
  <ContainerImageFormat>OCI</ContainerImageFormat>`
</PropertyGroup>