Switch docker base image to one with uv preinstalled

[cyberw]

Prerequisites

• I am using the latest version of Locust
• I am suggesting a new feature, not asking a question

Description

If someone (like @mquinnfd , but could be anyone :) feels like it, it would be nice to swap out the python:3.12-slim base Docker image for one that has uv from the start.

Probably, the best equivalent is ghcr.io/astral-sh/uv:debian-slim (https://docs.astral.sh/uv/guides/integration/docker/#available-images)

[github-actions]

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