Update Dockerfile to use Python 3.13 as base

Python 3.13 has been supported since version 2.32, but the base image was still based on 3.11.

Dockerfile

@@ -11,7 +11,7 @@ RUN yarn webui:install --production --network-timeout 60000
 RUN yarn webui:build
 
 # Stage 2: Build Locust package
-FROM python:3.11-slim AS base
+FROM python:3.13-slim AS base
 
 FROM base AS builder
 RUN apt-get update && apt-get install -y git