[feat:ci/cd] Add GitHub Actions for Automated Docker Image Building

# 🐳 Add GitHub Actions for Automated Docker Image Building

## Problem Statement

Currently, the project requires manual Docker image building and distribution. Users need to build images locally using `docker-compose`.

## Proposed Solution

Implement a comprehensive GitHub Actions workflow that automatically builds and publishes Docker images to GitHub Container Registry.

## Features to Add

### 🏗️ **Production Dockerfiles**
- [ ] `Dockerfile` - Standard CPU-only image with multi-architecture support (AMD64/ARM64)
- [ ] `Dockerfile.gpu` - GPU-enabled image with CUDA 11.8 support (AMD64 only)
- [ ] Optimized for production with security best practices

### ⚙️ **GitHub Actions Workflow**
- [ ] Automated building on push to main/develop branches
- [ ] Build testing on pull requests (without publishing)
- [ ] Multi-architecture builds (AMD64 + ARM64 for CPU images)
- [ ] Automatic tagging based on Git events:
- `latest` for main branch
- `develop` for develop branch
- `v1.2.3` for version tags
- `pr-123` for pull requests

### 📦 **Container Registry Integration**
- [ ] Publish to GitHub Container Registry (`ghcr.io`)
- [ ] Proper image tagging and versioning
- [ ] Layer caching for faster builds
- [ ] Security scanning integration

### 🚀 **Production Configurations**
- [ ] `docker-compose.prod.yml` using pre-built images
- [ ] `.env.prod.example` template for production deployments
- [ ] Health check endpoints for container monitoring

### 📚 **Documentation**
- [ ] Comprehensive build guide (`DOCKER_BUILD_GUIDE.md`)
- [ ] Usage examples and troubleshooting
- [ ] Architecture support matrix

## Benefits

1. **🚀 Faster Deployment**: Users can pull pre-built images instead of building locally
2. **🔄 Consistent Builds**: All images built in standardized GitHub environment
3. **🛡️ Security**: Production hardened images with non-root users
4. **📈 Scalability**: Multi-architecture support for different deployment targets
5. **🔧 Developer Experience**: Simplified setup for contributors and users
6. **🎯 CI/CD Ready**: Foundation for automated testing and deployment

## Technical Details

### Image Variants
- **Standard Image**: `ghcr.io/catchthetornado/text-extract-api:latest`
- Python 3.10 slim base
- CPU-only processing
- Multi-architecture (AMD64/ARM64)

- **GPU Image**: `ghcr.io/catchthetornado/text-extract-api-gpu:latest`
- NVIDIA CUDA 11.8 base
- GPU-accelerated processing
- AMD64 architecture only

### Workflow Triggers
- Push to `main` → Build and publish `latest` images
- Push to `develop` → Build and publish `develop` images
- Git tags (`v*`) → Build and publish versioned images
- Pull requests → Build and test (no publish)

### Security Features
- Non-root user execution (`appuser`)
- Minimal attack surface with slim base images
- Health check endpoints for monitoring
- Secure layer caching and optimization

## Implementation Checklist

- [ ] Create production `Dockerfile`
- [ ] Create GPU-enabled `Dockerfile.gpu`
- [ ] Implement GitHub Actions workflow (`.github/workflows/docker-build.yml`)
- [ ] Add health check endpoint to FastAPI application
- [ ] Create production Docker Compose configuration
- [ ] Add comprehensive documentation
- [ ] Update `.dockerignore` for optimized builds
- [ ] Create environment template for production
- [ ] Test workflow with sample builds

## Success Criteria

✅ **Automated Builds**: Images build automatically on code changes
✅ **Multi-Platform**: CPU images work on both Intel/AMD and Apple Silicon
✅ **GPU Support**: GPU images work with NVIDIA Docker runtime
✅ **Documentation**: Clear setup and usage instructions
✅ **Testing**: PR builds validate Docker configurations
✅ **Production Ready**: Images suitable for production deployment