Add documentation for required environment variables in Docker Compose

[Cinzya]

Summary

This PR adds comprehensive documentation for the required environment variables feature in Docker Compose files, addressing issue #345.

Changes

1. Main Docker Compose Documentation (docs/builds/packs/docker-compose.md)

• Added "Required Environment Variables" section explaining the :? syntax
• Included practical examples showing required, required-with-defaults, and optional variables
• Documented UI behavior (red borders for empty required fields, sorting, prefilled defaults)

2. Knowledge Base Docker Compose Page (docs/knowledge-base/docker/compose.md)

• Added comprehensive section on required environment variables
• Explained syntax and key behaviors
• Included information about validation and error handling

3. Service Contribution Guide (docs/get-started/contribute/service.md)

• Added guidance for template authors on using required variables
• Included example showing how to mark variables as required in service templates
• Helps contributors understand when to use required vs optional variables

Benefits

• Improved User Experience: Users can immediately see which configuration is essential
• Reduced Deployment Failures: Validation happens before container creation
• Better Service Templates: Template authors can clearly indicate critical configuration
• Clear Visual Feedback: Red borders and sorted display help users quickly identify required fields

Related Issue

Closes #345

Testing

The documentation has been reviewed for:

• Technical accuracy
• Consistency with existing documentation style
• Proper cross-references between related pages
• Clear and concise explanations

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]