Wait for successful airflow-init service completion

[mik-laj]

We had a race condition when the containers were started. All airflow-* containers were started simultaneously, but airflow-worker, airflow-scheduler and airflow-webserver require the database to be initialized first. This was sometimes done before the containers were started, but sometimes the container referred to a table that was not migrated. Now the database is always initialized before the rest of the containers are started.

Example: https://apache-airflow.slack.com/archives/CCQ7EGB1P/p1621711385211600
CC: @marclamberti
PS. I working on examples for other configuration: https://github.com/mik-laj/airflow-docker-compose-examples

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[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest master or amend the last commit of the PR, and push it with --force-with-lease.