chore(docker): bump base image from python:3.9-slim to python:3.12-slim

[jake1164]

Summary

• Python 3.9 is EOL and can't satisfy newer selenium (>=4.37) or pytest (>=9.0) releases, which require Python >=3.10.
• This is what's blocking dependabot PRs Update selenium requirement from >=4.9.0 to >=4.43.0 #9 (selenium >=4.43.0) and Update pytest requirement from <10.0.0,>=7.0.0 to >=9.0.3,<10.0.0 #12 (pytest >=9.0.3) from building.
• Bumps the Dockerfile base image to python:3.12-slim.

Test plan

• Built image locally with the new base image
• Ran full test suite in the container: 103 passed, 1 skipped

[Copilot]

Pull request overview

Updates the project’s container runtime to a supported Python version so Dependabot upgrades (notably newer selenium/pytest) can build and run successfully in the Docker-based environment.

Changes:

• Bump Docker base image from python:3.9-slim to python:3.12-slim.

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