fix: add celery beat service to docker compose

[chazzhou]

Description

This change adds a new beat service to the docker-compose.yaml file for the self-hosted version of Dify. The beat service is responsible for running scheduled tasks using Celery beat. Previously, the self-hosted version did not have the beat service running, and as a result, scheduled tasks in api/schedule listed in beat_schedule under api/extensions/ext_celery.py were not being executed.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Verified that the beat service starts successfully when running docker-compose up
• Confirmed that scheduled tasks are being executed as expected with the beat service running

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods
• optional I have made corresponding changes to the documentation
• optional I have added tests that prove my fix is effective or that my feature works
• optional New and existing unit tests pass locally with my changes

[JohnJyong]

the beat service is used for our SAAS env, so it‘s not necessary to add in the docker-compose.yaml
if you need this service , you are free to execute it locally， thanks @chazzhou

[bowenliang123]

This is an interesting hint for all local deployment users. Maybe celery beat should be mentioned on the docs for Local Source Code Start(https://docs.dify.ai/getting-started/install-self-hosted/local-source-code).