SDK Container label generation should use SOURCE_DATE_EPOCH instead of current date when using this package

[baronfel]

The SDK's container generation sets labels on the generated image(s). This includes a org.opencontainers.image.created and org.opencontainers.artifact.created label that uses DateTime.UtcNow. This isn't great for reproducibility.

This package should add a simple shim that uses SOURCE_DATE_EPOCH (or datetime.minvalue if not set) to ensure reproducibility.