Add Dockerfile for legacy Linux build environment with Ubuntu 18.04, Python 3.8, Node.js 16, and GCC 8

[Copilot]

This PR adds a complete Docker-based build environment for creating legacy Linux versions of electerm. The implementation provides a stable, reproducible build environment with specific versions required for legacy compatibility.

What's Added

Dockerfile.legacy-linux - A comprehensive Docker image based on Ubuntu 18.04 that includes:

• Python 3.8 with proper system integration via update-alternatives
• Node.js 16 installed from NodeSource repository for legacy compatibility
• GCC 8 configured as the default compiler
• Essential build tools and Electron application dependencies
• Non-root builder user for secure building practices

validate-environment.sh - A validation script that verifies:

• All tool versions are correctly installed
• Compiler functionality works properly
• Python and Node.js runtimes are operational
• System environment is ready for building

Updated README.md with comprehensive documentation covering:

• Build and usage instructions
• Environment validation procedures
• Tool specifications and versions
• Example commands for building electerm

.dockerignore for optimized build context and faster builds

Usage Example

# Build the legacy environment
docker build -f Dockerfile.legacy-linux -t electerm-builder-legacy .

# Run with mounted source code
docker run -it --rm -v /path/to/electerm:/workspace electerm-builder-legacy

# Validate the environment
./validate-environment.sh

# Build electerm
npm install && npm run build

Technical Details

The Dockerfile follows Docker best practices including:

• Multi-layer optimization with proper caching
• Security-first approach with non-root user
• Minimal attack surface by cleaning package caches
• Proper environment variable configuration
• Version pinning for reproducible builds

This provides a consistent, isolated build environment that eliminates "works on my machine" issues when building legacy Linux versions of electerm.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.