Add Docker stack example for Python venv

[Andrushika]

I added an example Dockerfile showing how to create a Python virtual environment in the Airflow Docker image and install packages from requirements.txt.

This helps users understand how to prepare a venv in a custom image and use it with ExternalPythonOperator or @task.external_python.

• closes: Add example of dockerfile with creating new virtualenv #26881

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Was generative AI tooling used to co-author this PR?

• Yes, generated-by: Codex GPT 5.5 following the guidelines

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[potiuk]

Nice!

[github-actions]

Hi maintainer, this PR was merged without a milestone set.
We've automatically set the milestone to Airflow 3.3.1 based on: backport label targeting v3-3-test
If this milestone is not correct, please update it to the appropriate milestone.

This comment was generated by Milestone Tag Assistant.

[github-actions]

Backport successfully created: v3-3-test

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
✅	v3-3-test