.<

Okay, so I haven't solved it. I'm using the file

{ lib
, fetchFromGitHub
, stdenvNoCC
, php83
, nixosTests
, dataDir ? "/var/lib/firefly-iii"
}:

let
  pname = "firefly-iii";
  version = "6.1.18";
  phpPackage = php83;

  src = fetchFromGitHub {
    owner = "firefly-iii";
    repo = "firefly-iii";
    rev = "v${version}";
    hash = "sha256-mA7gvKhHouUUz1Aix7253O/+VcufoEFwdcJeZxnazEo=";
  };
in

stdenvNoCC.mkDerivation (finalAttrs: {
  inherit pname src version;

  buildInputs = [ phpPackage ];

  nativeBuildInputs = [
    phpPackage.packages.composer
    phpPackage.composerHooks.composerInstallHook
  ];

  composerRepository = phpPackage.mkComposerRepository {
    inherit (finalAttrs) src pname version;
    composerNoDev = true;
    composerNoPlugins = true;
    composerNoScripts = true;
    composerStrictValidation = true;
    # composerLock = null;
    vendorHash = "sha256-EpMypgj6lZDz6T94bGoCUH9IVwh7VB4Ds08AcCsreRw=";
  };

  passthru = {
    inherit phpPackage;
    tests = nixosTests.firefly-iii;
  };
    # cp -a ${assets} $out/public

  postInstall = ''
    mv $out/share/php/${pname}/* $out/
    rm -R $out/share $out/storage $out/bootstrap/cache $out/public
    ln -s ${dataDir}/storage $out/storage
    ln -s ${dataDir}/cache $out/bootstrap/cache
  '';

  meta = {
    changelog = "https://github.com/firefly-iii/firefly-iii/releases/tag/v${version}";
    description = "Firefly III: a personal finances manager";
    homepage = "https://github.com/firefly-iii/firefly-iii";
    license = lib.licenses.agpl3Only;
    maintainers = [ lib.maintainers.savyajha ];
  };
})