feat: add Celery healthcheck files

[puehringer]

Developer Checklist (Definition of Done)

Issue

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Summary of changes

• Health checking celery is not so easy, so we use files created on startup to check if it is still alive: [K8S] livenessProbe and readinessProbe for celery beat and workers celery/celery#4079 (comment)

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Thanks for creating this pull request 🤗