Is there a way to start devenv-up of something other than the default shell?

[ElrohirGT]

Hi! For context I'm using devenv with flakes, here's an MRE of my situation:

{
  description = "Basic MRE";

  inputs = {
    nixpkgs.url = "github:cachix/devenv-nixpkgs/rolling";
    systems.url = "github:nix-systems/default";
    devenv.url = "github:cachix/devenv";
    devenv.inputs.nixpkgs.follows = "nixpkgs";
  };

  nixConfig = {
    extra-trusted-public-keys = "devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw=";
    extra-substituters = "https://devenv.cachix.org";
  };

  outputs = {
    self,
    nixpkgs,
    devenv,
    systems,
    ...
  } @ inputs: let
    forEachSystem = nixpkgs.lib.genAttrs (import systems);
    postgresHost = "127.0.0.1";
    postgresPort = 5566;
  in {
    packages = forEachSystem (system: {
      devenv-up = self.devShells.${system}.default.config.procfileScript;
      devenv-up-ci = self.devShells.${system}.ci.config.procfileScript;
    });

    devShells =
      forEachSystem
      (system: let
        pkgs = import nixpkgs {inherit system;};
        dbInitFile = "CREATE DATABASE db_test;";
      in {
        ci = devenv.lib.mkShell {
          inherit inputs pkgs;
          modules = [
            {
              packages = with pkgs; [
                oxlint
              ];

              languages.javascript = {
                enable = true;
                yarn.enable = true;
              };

              process = {
                process-compose = pkgs.lib.mkOptionDefault {
                  tui = "false";
                };
              };

              processes = {
                # Start the backend...
                backendApi = {
                  exec = "tail -f /dev/null";
                  process-compose = {
                    depends_on = {
                      postgres = {
                        condition = "process_started";
                      };
                    };
                  };
                };
              };

              services.postgres = {
                enable = true;
                listen_addresses = postgresHost;
                port = postgresPort;
                initialScript = dbInitFile;
              };
            }
          ];
        };

        default = devenv.lib.mkShell {
          inherit inputs pkgs;
          modules = [
            {
              packages = with pkgs; [
                dprint # Javascript formatter
                oxlint # Javascript linter
                act # run github actions locally
              ];

              languages.javascript = {
                enable = true;
                yarn = {
                  enable = true;
                };
              };

              services.postgres = {
                enable = true;
                listen_addresses = postgresHost;
                port = postgresPort;
                initialScript = dbInitFile;
              };
            }
          ];
        };
      });
  };
}

How would I go about starting the services defined in the ci shell? I tried running:

nix develop --impure .#ci --command bash -c "devenv up"

But this still runs only the postgres service thats defined in the default shell.

I also tried running:

nix run .#devenv-up-ci

But this fails with:

error:
       … while evaluating the attribute 'devShells.x86_64-linux.ci.config.procfileScript'

         at /nix/store/2r2766i4xifvn5s2wwysw6k9gz3d6c3i-source/flake.nix:38:9:

           37|       in {
           38|         ci = devenv.lib.mkShell {
             |         ^
           39|           inherit inputs pkgs;

       … in the left operand of the update (//) operator

         at /nix/store/zslv4c6fawyd1bj84a2i4x14gblz3h1w-source/flake.nix:165:24:

          164|           in
          165|           config.shell // {
             |                        ^
          166|             ci = config.ciDerivation;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Failed assertions:
       - devenv was not able to determine the current directory.

         See https://devenv.sh/guides/using-with-flakes/ how to use it with flakes.