Multiarch docker images?

[ismith]

It'd be nice to have images for arm64 in addition to x86_64.

I've got a fork+PR that do that, at https://github.com/ismith/kspan/pull/2. Would there be interest in having that here?

Components of the change:

• instead of using docker to build, use ko. Could put this in the Makefile, but it'd eliminate the need for a Dockerfile
• my PR publishes on tagging to the GHCR, not dockerhub, but if this repo has access to your dockerhub creds, we could fix that