[open-swe] feat: Add comprehensive environment variable configuration for Alpine Linux development

[dtnxlc]

Fixes #2

This PR expands the sample Alpine Linux development environment with comprehensive environment variable configuration to improve developer experience and environment consistency.

Changes Made

• Created .env.example with common Alpine Linux development environment variables including system configuration (ALPINE_VERSION, TIMEZONE, LANG), shell settings (EDITOR, SHELL), and development variables (NODE_ENV, DEBUG, LOG_LEVEL)
• Created .env with working default values for immediate use
• Updated .devcontainer/devcontainer.json to load environment variables from .env file and configured common development tools via features
• Enhanced README.md with detailed documentation on environment variables, their purposes, and customization instructions
• Added .env to .gitignore to prevent committing local configurations while keeping .env.example tracked

Benefits

• Provides consistent development environment setup across different machines
• Includes sensible defaults that work out-of-the-box
• Allows easy customization through environment variables
• Follows best practices for environment configuration management
• Improves developer onboarding experience