Use an init process as container entrypoint

[Kytha]

Overview

Our image runs the entry point script directly as the container entry point. This is against docker best practices because it omits the use of an init process. This will mean that no zombie processes will get properly reaped and will accumulate until system hits process limit. This is a problematic issue for a software like airflow, that dynamically creates subprocesses regularly.

Instead, our image should follow airflow and docker best practices and make use of a lightweight init process like dumb-init or tini.

This issue may be the reason why we sometimes see issues related to delays in worker task shutdown, or schedulers stalling after long lifetimes.

Acceptance Criteria

• Integrate a lightweight init process to all airflow images

Additional Info

Also see related discussion in github airflow issue: apache/airflow#25225