[AIRFLOW-8605] Added docker compose

[habibdhif]

This adds a docker compose example to run Airflow installation.

The Compose defines multiple services to run Airflow. There is an init service which is an ephemeral container to initialize the database and creates a user if necessary. The init service command tries to run airflow list_users and if it fails it initializes the database and creates a user. Different approaches were considered but this one is simple enough and only involves airflow commands (no database-specific commands).

Extension fields are used for airflow environment variables to reduce code duplication.

I added some lines of documentation on IMAGES.rst but maybe there is a better place for that.

Issues: #8605 #8606

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[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/master/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, pylint and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it’s a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Be sure to read the Airflow Coding style.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: [email protected]
  Slack: https://apache-airflow-slack.herokuapp.com/

[mik-laj]

Should we add this documentation to docs/*.rst? Otherwise this documentation will not be available to end users.

[BasPH]

Note there's another PR for a docker-compose file: #8621

[habibdhif]

Note there's another PR for a docker-compose file: #8621

Thanks for notifying me, closing the PR and moving this example under the #8605 issue.