fix(standalone): remove hardcoded webserver-port

[boredland]

Port 8080 is the default port for webserver (https://airflow.apache.org/docs/apache-airflow/stable/cli-and-env-variables-ref.html?highlight=webserver#webserver). By setting it here again explicitly, we forbid users to override it using AIRFLOW__WEBSERVER__WEB_SERVER_PORT. Removing it IMO is not a breaking change, since it will still default to 8080.

I tested locally if it still spins up the web server at port 8080. It did.

TODO:

• add to docs how to override port

[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)
Here are some useful points:

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[boredland]

add to docs how to override port

as we spoke about what I as a user would have expected: I found the configuration variables page and expected them to "just work". So from my perspective a hint to that page is even more than enough. Let me know if you'd like me to add more.

For people stumbling upon this as long as it isn't merged/released and like me are using standalone in a docker context, you can extend from the image and apply this tiny patch:

FROM apache/airflow:2.2.2-python3.9

## All airflow.cfg options can be set via environment variables
## See: https://airflow.apache.org/docs/apache-airflow/stable/configurations-ref.html
ENV AIRFLOW__WEBSERVER__WEB_SERVER_PORT=${PORT:-5000}

COPY app.json ./

# TODO: remove when https://github.com/apache/airflow/pull/20429 is merged and released
RUN sed -i -e 's/, "--port", "8080"//g' /home/airflow/.local/lib/python3.9/site-packages/airflow/cli/commands/standalone_command.py

CMD ["airflow", "standalone"]

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!