Several SPDX SBOM adjustments. (#760)
Several SPDX SBOM adjustments. (#760)

1. Change index -> image relationship type from `CONTAINS` to `VARIANT_OF` (I think this was an oversight in my original PR),
2. Always include `mediaType` in pURLs for index/images we produce (I'm not adding this to the base image, since it's not readily available, but we can add it there if we want to find a way to plumb it through),
3. Include more platform discriminator information to the pURLs we use in index -> image.
ko-build/ko,ko-build/ko