Try a different tack...

objectifier.nix

@@ -1,12 +1,33 @@
 { pkgs, lib, ... }:
 
-pkgs.stdenv.mkDerivation {
+let
+  objectifier-src = pkgs.stdenv.mkDerivation {
+    name = "objectifier";
+    src = ./src;
+    phases = [ "installPhase" ];
+    installPhase = ''
+      mkdir -p $out
+      cp $src/detector.py $out/detector.py
+      cp $src/yolo-cli.py $out/objectifier.py
+    '';
+  };
+
+  pythonYolo = pkgs.python3.withPackages (pyPkgs:
+    with pyPkgs; [
+      gunicorn
+      fastapi
+      gunicorn
+      opencv4
+      python-multipart
+      uvicorn
+    ]);
+in pkgs.writeShellApplication {
   name = "objectifier";
-  src = ./src;
-  phases = [ "installPhase" ];
-  installPhase = ''
-    mkdir -p $out
-    cp $src/detector.py $out/detector.py
-    cp $src/yolo-cli.py $out/objectifier.py
-  '';
+  runtimeInputs = [ pythonYolo ];
+  text = pkgs.lib.concatStringsSep " " [
+    "gunicorn"
+    "objectifier:app"
+    "-k uvicorn.workers.UvicornWorker"
+    ''"$@"''
+  ];
 }