Nix feature should support `"flakeUri": "../flake.nix"` to local file instead of remote Nix URI

[vorburger]

https://github.com/devcontainers/features/tree/main/src/nix#location-of-flakes :

Currently flakeUri works best with a remote URI (e.g., github:nixos/nixpkgs/nixpkgs-unstable#hello) as local files need to be in the image.

makes the Nix feature of fairly limited use for dev containers in Nix package style repos which contain a flake.nix (e.g. like this one; for [my] enola-dev/enola#1921).

I've tried both "flakeUri": "../flake.nix" and "flakeUri": "flake.nix" but they both fail with error: getting status of '/tmp/dev-container-features/flake.nix': No such file or directory, because the base URL is wrong.

Given that https://github.com/devcontainers/features/tree/main/src/nix#location-of-flakes also says:

Proposed support for lifecycle hooks in Features (devcontainers/spec#60) would allow for expressions files or Flakes to exist in the source tree to be automatically installed on initial container startup

and seeing that devcontainers/spec#60 is now implemented, perhaps support for this could now be added?

@Chuxel

[vorburger]

Would simply running nix develop in a devcontainer be equivalent?

Which "phase" would be appropriated? An onCreateCommand or a postAttachCommand (or nix develop --command bash, as e.g. here ?)

The thing is, at least in my project, the Nix flake's packages have to be on the PATH not just of the Shell in the Terminal, but also already for "the VSC server environment" (?) so that VSC Extensions also have access to various such packages. What's the correct way of doing this?

The simplest would IMHO be if flakeUri would just directly support local flake.nix files.