ARM Images slow to build on default runners

[scottyhq]

In #561 we started creating images for ARM platforms. I think it's known that using Docker QEMU is slow, but I didn't realize this would take 3-4x longer.

Anyways there are probably many ways to optimize this

• https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/
• https://stackoverflow.com/questions/66337210/is-it-possible-to-push-docker-images-for-different-architectures-separately

[scottyhq]

ARM runners are now free for public repositories:

https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/

[weiji14]

Some instructions available at docker/docs#21973 on how to distribute builds across runners with matching platform/arch.

Slightly related topic, should we start to use bake as you've mentioned at #399 (comment) and https://github.com/scottyhq/pangeo-buildx? It looks like repo2docker is using buildkit now with jupyterhub/repo2docker#875 and jupyterhub/repo2docker#1402.