Summary

• Adds a new Coolify service template for Docker Mailserver, a full-stack self-hosted email server
• Includes SMTP/IMAP ports, DKIM, DMARC, SpamAssassin, and spoof protection
• Persistent volumes for mail data, state, logs, and configuration
• Template file: templates/compose/docker-mailserver.yaml

Closes #8266 /claim #8266

Test plan

• Deploy the template in Coolify and verify the mail server starts
• Verify SMTP healthcheck passes
• Verify mail ports (25, 143, 465, 587, 993) are exposed correctly

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