[WIP] Build ARM64 Docker images

[manics]

We're doing it in JupyterHub and Z2JH, why not here too?

This is a modification of the JupyterHub workflow. The main difference is the Docker image tags are calculated using the method from the existing Docker Hub automated build hook: https://github.com/jupyterhub/repo2docker/blob/eb7520901913d61e7745d136ce127142d5fa8506/hooks/post_push#L3-L6

Prerequisites:

• [ ] Disable automated Docker Hub build
• [ ] Add secrets.DOCKERHUB_USERNAME secrets.DOCKERHUB_TOKEN to push to Docker Hub
• [ ] Figure out how to freeze packages for arm64 (https://github.com/jupyterhub/repo2docker/pull/1024 makes it a lot easier)

[minrk]

Figure out how to freeze packages for arm64 (Refreeze with conda-lock #1024 makes it a lot easier)

I believe this will mostly be adding --platform linux-aarch64 --platform linux-64 to the lock call (and dealing with any issues if there are unsupported packages or versions).

Then it's mostly picking the right lockfile for the architecture in the CondaBuildPack. I'm not sure where that should come from. Explicit arg only, or detect platform? I'm not sure where else we assume x64.

[consideRatio]

This is a PR related to #1180