feat: allow pass hostname in docker env

[fatelei]

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Summary

fix #30957

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @fatelei, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the SMTP client functionality by allowing users to define a specific local hostname for outgoing email connections via an environment variable. This feature provides greater control over how the application identifies itself to mail servers, which can be beneficial for compatibility with certain SMTP server configurations or for meeting specific network requirements. The change is implemented by reading SMTP_LOCAL_HOSTNAME and passing it to the underlying Python smtplib calls, with corresponding updates to the unit tests to validate this new parameter.

Highlights

• SMTP Local Hostname Configuration: Introduced the ability to specify a local hostname for SMTP connections by reading the SMTP_LOCAL_HOSTNAME environment variable. This value is then passed to the smtplib.SMTP and smtplib.SMTP_SSL constructors.
• Environment Variable Integration: The api/libs/smtp.py file now imports the os module to retrieve the SMTP_LOCAL_HOSTNAME environment variable, allowing dynamic configuration of the local hostname for outgoing emails.
• Unit Test Updates: Updated existing unit tests in api/tests/unit_tests/libs/test_smtp_client.py and api/tests/unit_tests/tasks/test_mail_send_task.py to account for the new local_hostname parameter being passed to the smtplib constructors, ensuring the new functionality is correctly tested.

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[gemini-code-assist]

Code Review

This pull request introduces the functionality to specify a local hostname for SMTP connections via the SMTP_LOCAL_HOSTNAME environment variable. The implementation is correct and the accompanying test updates are appropriate. I have a couple of suggestions to improve maintainability and consistency with the existing configuration handling.

[QuantumGhost]

The .env.example in api folder should be updated, too.