throw "USER environment variable is not set. Run with --impure and ensure USER is exported."

[vorburger]

In a Codespace I'm using with Nix, this doesn't quite work... even though the Nix based Container Test CI passes:

2026-03-21 08:59:56.538Z: copying path '/nix/store/z4bfwjlbd6bhasl6hndq9wi6y7xfsbqg-home-manager-0-unstable-2026-03-13' from 'https://cache.nixos.org'...

2026-03-21 08:59:57.906Z: error:
       … while calling the 'throw' builtin
         at /nix/store/2fhrvwyzf7jsky5lg3532agdpv91554x-source/dotfiles/home-manager/flake.nix:26:11:
           25|         if u != "" then u else
           26|           throw "USER environment variable is not set. Run with --impure and ensure USER is exported.";
             |           ^
           27|       envHOME = builtins.getEnv "HOME";

       error: USER environment variable is not set. Run with --impure and ensure USER is exported.

2026-03-21 08:59:57.997Z: error:
       … while calling the 'throw' builtin
         at /nix/store/2fhrvwyzf7jsky5lg3532agdpv91554x-source/dotfiles/home-manager/flake.nix:26:11:
           25|         if u != "" then u else
           26|           throw "USER environment variable is not set. Run with --impure and ensure USER is exported.";
             |           ^
           27|       envHOME = builtins.getEnv "HOME";

       error: USER environment variable is not set. Run with --impure and ensure USER is exported.

2026-03-21 08:59:58.077Z: error:
       … while calling the 'throw' builtin
         at /nix/store/2fhrvwyzf7jsky5lg3532agdpv91554x-source/dotfiles/home-manager/flake.nix:26:11:
           25|         if u != "" then u else
           26|           throw "USER environment variable is not set. Run with --impure and ensure USER is exported.";
             |           ^
           27|       envHOME = builtins.getEnv "HOME";

       error: USER environment variable is not set. Run with --impure and ensure USER is exported.

2026-03-21 08:59:58.160Z: error:
       … while calling the 'throw' builtin
         at /nix/store/2fhrvwyzf7jsky5lg3532agdpv91554x-source/dotfiles/home-manager/flake.nix:26:11:
           25|         if u != "" then u else
           26|           throw "USER environment variable is not set. Run with --impure and ensure USER is exported.";
             |           ^
           27|       envHOME = builtins.getEnv "HOME";

       error: USER environment variable is not set. Run with --impure and ensure USER is exported.

2026-03-21 08:59:58.171Z: 
2026-03-21 08:59:58.171Z: 
2026-03-21 08:59:58.171Z: Exit code 1
2026-03-21 08:59:58.174Z: Stop: Run in container: # Clone & install dotfiles
2026-03-21 08:59:58.187Z: Outcome: success User: code WorkspaceFolder: /workspaces/enola2
2026-03-21 08:59:58.194Z: devcontainer process exited with exit code 0

=================================================================================
2026-03-21 08:59:58.195Z: Finished configuring codespace.

[vorburger]

The repo where this happens has this, which might have something to do with it, and its Dockerfile:

FROM ghcr.io/xtruder/nix-devcontainer:v1

# cache /nix
VOLUME /nix

and in its devcontainer.json:

{
  "name": "...",
  "dockerFile": "Dockerfile",
  "context": "${localWorkspaceFolder}",
  "build": {
    "args": {
      "USER_UID": "${localEnv:USER_UID}",
      "USER_GID": "${localEnv:USER_GID}"
    }
  },
  // run arguments passed to docker
  "runArgs": [
    "--security-opt",
    "label=disable"
  ],
  "containerEnv": {
    // extensions to preload before other extensions
    "PRELOAD_EXTENSIONS": "arrterian.nix-env-selector"
  },
  // disable command overriding and updating remote user ID
  "overrideCommand": false,
  "userEnvProbe": "loginShell",
  "updateRemoteUserUID": false,
  "onCreateCommand": "nix develop --command bash -c 'echo done building nix dev environment'",
  },
  "mounts": [
    "source=/var/run/docker.sock,target=/var/run/docker.sock,type=bind"
  ],
}