Update environment flask-server

Setting up Python environment with Flask

.container-use/AGENT.md

@@ -1 +1 @@
-No instructions found. Please look around the filesystem and update me
+This environment contains a simple Flask server that serves a basic web application. To run it, use `python app.py`.

.container-use/environment.json

@@ -1,4 +1,7 @@
 {
   "workdir": "/workdir",
-  "base_image": "ubuntu:24.04"
+  "base_image": "python:3.9-slim",
+  "setup_commands": [
+    "pip install flask"
+  ]
 }