weaveworks/ignite:<version>-arm64 images have amd64 as their architecture

[twelho]

This doesn't affect weaveworks/ignite:<version> images, those are registered for both architectures. Just a nit, but would be good to fix at some point.

[stealthybox]

To clarify, our manifest list references the -arm64 image properly, but the image itself has metadata for the wrong OS:

docker manifest inspect weaveworks/ignite:v0.6.2-arm64 -v
...
		"platform": {
			"architecture": "amd64",
			"os": "linux"
		}

This is because the artifact is cross-compiled in a multi-stage build and constructed on an amd64 host.
This doesn't break usability for arm hosts, but it's a bit inaccurate depending on how you view things.

---

We also have another subtle issue.
Our manifest list currently specifies:

docker buildx imagetools inspect weaveworks/ignite:v0.6.2
...
  Name:      docker.io/weaveworks/ignite:v0.6.2@sha256:c9b04a099a62fc44e3a26b4e098eabeb9dc34587205d3aa16e1cbc009f1a4f8b
  Platform:  linux/amd64
             
  Name:      docker.io/weaveworks/ignite:v0.6.2@sha256:41395728d971f2e46490393076e26b4f7351ed67a2e1b62cf2b2dd2b187664fc
  Platform:  linux/arm64

The second descriptor should probably also specify the arm64 variant:
ie: linux/arm64/v8

Compare ubuntu:latest with our os/arch/variant:

• https://hub.docker.com/r/weaveworks/ignite/tags?page=1&name=v0.6.2
• https://hub.docker.com/_/ubuntu?tab=tags&page=1&name=latest