Add trigger process to Airflow Docker docs

[amarlearning]

closes: #29150

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
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[potiuk]

I like it. It is also future-proof

[boring-cyborg]

Awesome work, congrats on your first merged pull request!