[WIP] Use flake to push to ghcr.io with multiplatform images

[lucernae]

Hi team,
I needed a native arm images for my M1 devcontainer, so I forked the repo and made a small PoC in my fork.

This is probably useable in upstream repo as well.
Can someone help decide if this PR is worth to be merged/follow up?

The general idea is to use flake.nix to build a matrix of images with a combination of nix channel x nix system.
For each image per system, we push to ghcr.io.
For each images, we created multiplatform manifests that combines each system per images. This way, user can use a single unified tag and docker daemon will pick architecture that fits best, just like buildx. I used this approach because I don't know how to directly create multiarch images from nix dockertools.

I've tested this PoC for bash and devcontainer in my own repo.

Thanks for the comments!