Add Docker Mailserver template for email server stack

[ccc196884]

Summary

Adds a reusable Docker Mailserver template for managing client email domains as requested in issue #8266.

Changes

• ✅ Clean compose template - No comments (addresses maintainer feedback from PR Add one-click service template for Docker Mailserver. #8275)
• ✅ Production-ready - Based on official docker-mailserver/docker-mailserver
• ✅ Essential features - SMTP, IMAP, LDAP, Antispam, Antivirus support
• ✅ Persistent volumes - Mail data, state, logs, and configuration
• ✅ Coolify compatible - Works with proxy and certificate management
• ✅ Configurable - Comprehensive environment variables for customization

Technical Details

• Uses official ghcr.io/docker-mailserver/docker-mailserver:latest image
• Exposes standard mail ports: 25 (SMTP), 143 (IMAP), 465/587 (ESMTP), 993 (IMAPS), 4190 (ManageSieve)
• Includes bind mounts for configuration files (accounts, virtual domains, etc.)
• Implements proper health checks and restart policies
• Supports Let's Encrypt SSL by default

Addresses Feedback

This implementation specifically addresses the maintainer feedback from PR #8275:

"Compose file comments are removed by Coolify, so submit a documentation PR instead"

• ❌ No comments in compose file (Coolify removes them anyway)
• ✅ Clean, minimal template focused on functionality
• ✅ Documentation references official project docs

Closes #8266

Provides the requested "reusable email server stack/template for managing client email domains" with full Docker Mailserver functionality.