It will use Nix to build a shell on your local machine directly, so you can run the program on your system directly (not in a VM).

It also has some early functionality where you can turn that shell into a Docker container, so you can run the same shell + program in other environments or in the cloud

so an abstration on top of docker?

It uses the Docker CLI to build container images out of the Nix shells it creates, if you ask it to export one to a Docker image for you. Otherwise it just uses Nix locally, no virtualization.

I wonder if it takes this approach because there's some issue with using Nixpkgs' dockerTools on macOS— those tools let you create Docker/OCI images without even having Docker installed.

https://nix.dev/tutorials/building-and-running-docker-images... asserts that you'd need a remote builder or cross-compiling. So it seems possible (unless maybe there are still enough cross-compile issues?)

Can cross builds hit the main binary caches or is that awaiting Hydra support for content-addressed derivations? Are there big caches for cross-compiled packages?

I'm guessing awaiting based on how much rebuilding I had to do yesterday.

docker is not virtualization either

1. Yes it is— there are many kinds of virtualization other than machine virtualization. Containers are a form of operating system virtualization.

2. Docker on non-Linux also requires machine virtualization; the `docker` CLI connects to a `dockerd` daemon running on a virtual machine.

The question I posed earlier is motivated by understanding this and knowing that on macOS, the binaries that devbox runs on your local machine are completely different (of a different architecture, even) than the binaries that devbox will pack into a Docker image. Since it uses Nix to build them, it either needs to run Nix in a virtual Linux guest (which it does using the guest that Docker for Mac sets up) or have its Mac-bound Nix build Linux binaries.

Using `docker buildx` obviates the need to set up devbox's own Linux VM to target as a remote builder, or, alternatively, cross compile the required Nix packages. But it comes at the cost of adding a dependency on Docker even though devbox doesn't use the Docker runtime for its shells

No, it just creates images compatible with docker.