Add CLI check for scheduler(s)

[mik-laj]

Part of: #11161

I miss a simple method to check if the scheduler is in good health, so I added a command that allows me to do it. Additionally, I updated docker-compose.yaml to take use it

For now, we've been doing without this command because we had a Python script implemented that did similar behavior, but that doesn't look right.

airflow/chart/templates/scheduler/scheduler-deployment.yaml

Lines 126 to 143 in 7d18150

	- python
	- -Wignore
	- -c
	- |
	import os
	os.environ['AIRFLOW__CORE__LOGGING_LEVEL'] = 'ERROR'
	os.environ['AIRFLOW__LOGGING__LOGGING_LEVEL'] = 'ERROR'
	from airflow.jobs.scheduler_job import SchedulerJob
	from airflow.utils.db import create_session
	from airflow.utils.net import get_hostname
	import sys
	with create_session() as session:
	job = session.query(SchedulerJob).filter_by(hostname=get_hostname()).order_by(
	SchedulerJob.latest_heartbeat.desc()).limit(1).first()
	sys.exit(0 if job.is_alive() else 1)

I am not updating the Helm Chart yet to keep compatibilityy with Airflow 1.10, but when we drop support for Aiirflow 1.10, we will be able to use it.

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[turbaszek]

The return value is either bool or an int - is this inconsistency expected?

[mik-laj]

Fixed. Thanks

[turbaszek]

What is the second method? This section (as defined by ====) mentions only API endpoint

[mik-laj]

Fixed. Thanks

[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

[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 master or amend the last commit of the PR, and push it with --force-with-lease.