Install Docker Python packages in an isolated venv

[dicnunz]

Summary

• install Docker Python dependencies into /opt/training-modules-python instead of the system interpreter
• expose that environment through VIRTUAL_ENV and PATH for all later Dockerfile steps and container users
• add python3-venv so the image can create the environment without --break-system-packages

Closes #885.

Verification

• git diff --check
• docker buildx build --check .
• python3 -m venv /tmp/training-modules-venv-check && /tmp/training-modules-venv-check/bin/python -m pip --version && rm -rf /tmp/training-modules-venv-check

Note: I did not run a full Docker build locally because this image compiles/downloads the full workshop stack. The Dockerfile static build check passes and covers Dockerfile parsing/build checks.

[dicnunz]

Small paid-fix note: if this closes the Docker Python packaging issue and saves maintainer time, I am asking $10 here: https://nicdunz.gumroad.com/l/slop-kill-pass?wanted=true

No issue if this repo only handles volunteer contributions; the PR is intended to stand on its own.

[dicnunz]

Rebased this onto current master (b614eaf) and force-pushed head b272c88.

The PR diff is still limited to the Dockerfile Python package install path:

• install python3-venv
• create /opt/training-modules-python
• put that venv on PATH
• install requirements.txt inside the venv instead of using --break-system-packages

Validation after rebase:

• git diff --check origin/master..HEAD

I did not rerun a full Docker build locally because that is heavier than this branch-only refresh, but the rebased diff is unchanged in scope.