Update Docker Compose documentation for SERVICE_URL magic environment variables and service naming

[yipfram]

Summary

This PR updates the Docker Compose documentation to clarify the usage of Coolify's magic environment variables, specifically the difference between SERVICE_URL_* and SERVICE_FQDN_*, and adds important information about service naming practices.

Changes Made

• Updated the URL type description to clarify that SERVICE_URL_SERVICE should be preferred for generating URLs for services
• Added clear examples showing the difference between SERVICE_URL_APPWRITE and SERVICE_FQDN_APPWRITE:
  • SERVICE_URL_APPWRITE: 'http://appwrite-vgsco4o.example.com' (full URL with protocol)
  • SERVICE_FQDN_APPWRITE: appwrite-vgsco4o.example.com (domain only)
• Updated the YAML example to consistently use SERVICE_URL_* instead of SERVICE_FQDN_* for URL generation
• Updated comments in the example to reflect the new logic
• Added service naming guidance: Added an info box explaining that service names with underscores (_) cannot use ports in environment variables, and recommends using hyphens (-) instead

Files Changed

• docs/knowledge-base/docker/compose.md

Testing

• Documentation builds successfully
• Changes follow the existing documentation style and structure

Context

This update addresses the recent changes in Coolify where SERVICE_URL_* environment variables are now the preferred method for generating complete URLs with protocols, while SERVICE_FQDN_* variables provide just the domain portion.

The service naming guidance addresses issues where services with underscores in their names cannot properly use port specifications in environment variables, as discussed in coollabsio/coolify#6152.

[yipfram]

I tried to do the change for URL to FQDN, please tell me if i understood the thing correctly

[yipfram]

Not sure about the hyphens thing, let me know...