Add devcontainer with Python 3.13 and uv support

Adds a Dockerfile-based development container using Microsoft's Python 3.13 image, with uv installed for dependency management.

Updates:
- Adds .devcontainer/Dockerfile based on Python 3.13
- Modifies devcontainer.json to use a local build
- Updates on-create-command.sh to install dependencies using uv

.devcontainer/devcontainer.json

@@ -1,6 +1,8 @@
 {
   "name": "pallets/flask",
-  "image": "mcr.microsoft.com/devcontainers/python:3",
+  "build": {
+    "dockerfile": "Dockerfile"
+  },
   "customizations": {
     "vscode": {
       "settings": {