Document upstream diff: our arch-specific images are `golang:1.16.6-buster-amd64`, not `amd64/golang:1.16.6-buster`

So far in my implementation of #11, arch-specific images are e.g. golang:1.16.6-buster-amd64, not amd64/golang:1.16.6-buster. (https://hub.docker.com/r/amd64/golang/)

We need this naming diff for a few reasons:

• Convention: In MCR, multi-arch tags use a single repo.
  • "Repo" being the golang part. amd64/golang is a separate repo.
• Infra limit: The ImageBuilder (.NET Docker infra) manifest.json schema is defined based on the MCR convention. Its structure means that for ImageBuilder to produce a multi-arch tag, the tags must follow the MCR convention. (A "shared tag" can only be defined within a single repo.)

I'm filing this issue to track documenting this diff vs. upstream while the reasons for it are fresh in my mind. 🙂 It doesn't seem like we need to fix this difference: as far as I know, there isn't a good reason our users would need the upstream repo/tag naming.

Start by locating the documentation covering arch-specific image tags and compare its naming with the examples in this issue. Document that the Microsoft images use golang:1.16.6-buster-amd64 rather than amd64/golang:1.16.6-buster, including the MCR convention and ImageBuilder manifest constraint; done when the upstream difference and rationale are recorded.