Fix Docker build: add --system to uv pip uninstall opencv-python

[nikolauspschuetz]

Description

Adds the missing --system flag to the uv pip uninstall opencv-python step in the Dockerfile.

Every other uv pip call in that RUN already passes --system, but the uninstall does not. Recent uv versions require --system for uninstall too when operating outside a virtual environment, so the Docker build now fails at that step:

error: No virtual environment found; run `uv venv` to create an environment,
or pass `--system` to install into a non-virtual environment

This breaks make docker-cpu / make docker-gpu and the docs image for anyone building with a current uv. The .[extra,tests,docs] install itself succeeds — only the subsequent uninstall fails.

Motivation and Context

The Docker image build is currently broken with recent uv. One-line fix, verified by building the image end-to-end (docker build . succeeds; import stable_baselines3 and the vec-env tests run in the resulting image).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (docs/misc/changelog.md) (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

_{Disclosure (per CONTRIBUTING): found and fixed with AI assistance while building the dev image; the diagnosis (missing --system on the uninstall) was verified by reproducing the failing build and rebuilding green.}