Add comprehensive Docker documentation

Co-authored-by: oovaa <103980029+oovaa@users.noreply.github.com>

DOCKER.md

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+# Docker Deployment Guide
+
+This guide explains how to run StackRender using Docker.
+
+## Quick Start
+
+### Using Docker Compose (Recommended)
+
+The easiest way to run StackRender:
+
+```bash
+docker-compose up -d
+```
+
+The application will be available at `http://localhost:8080`
+
+To stop the application:
+
+```bash
+docker-compose down
+```
+
+### Using Docker CLI
+
+Build the image:
+
+```bash
+docker build -t stackrender .
+```
+
+Run the container:
+
+```bash
+docker run -d -p 8080:80 --name stackrender stackrender
+```
+
+Stop and remove the container:
+
+```bash
+docker stop stackrender
+docker rm stackrender
+```
+
+## Configuration
+
+### Port Mapping
+
+By default, the application runs on port 80 inside the container and is mapped to port 8080 on the host. You can change this in `docker-compose.yml`:
+
+```yaml
+ports:
+  - "YOUR_PORT:80"
+```
+
+Or when using Docker CLI:
+
+```bash
+docker run -d -p YOUR_PORT:80 --name stackrender stackrender
+```
+
+### Health Check
+
+The Docker Compose configuration includes a health check that verifies the application is responding correctly:
+
+```yaml
+healthcheck:
+  test: ["CMD", "wget", "--quiet", "--tries=1", "--spider", "http://localhost:80/"]
+  interval: 30s
+  timeout: 10s
+  retries: 3
+  start_period: 40s
+```
+
+## Architecture
+
+### Multi-Stage Build
+
+The Dockerfile uses a multi-stage build approach:
+
+1. **Builder Stage**: Uses Node.js 20 to install dependencies and build the application
+2. **Production Stage**: Uses Nginx Alpine to serve the static files
+
+This approach minimizes the final image size by excluding build tools and dependencies.
+
+### Nginx Configuration
+
+The application uses a custom Nginx configuration (`nginx.conf`) that:
+
+- Enables gzip compression for better performance
+- Sets security headers (X-Frame-Options, X-Content-Type-Options, X-XSS-Protection)
+- Handles client-side routing (React Router)
+- Configures caching for static assets
+
+## Troubleshooting
+
+### Build Issues
+
+If you encounter SSL certificate issues during the build, the Dockerfile includes a workaround:
+
+```dockerfile
+RUN npm config set strict-ssl false && npm install
+```
+
+This is necessary in some build environments with certificate validation issues.
+
+### Container Not Starting
+
+Check the logs:
+
+```bash
+docker logs stackrender
+```
+
+Or with Docker Compose:
+
+```bash
+docker-compose logs
+```
+
+### Port Already in Use
+
+If port 8080 is already in use, you can change it in `docker-compose.yml` or use a different port with Docker CLI:
+
+```bash
+docker run -d -p 8081:80 --name stackrender stackrender
+```
+
+## Development
+
+For development, it's recommended to run the application directly with Node.js instead of Docker:
+
+```bash
+npm install
+npm run dev
+```
+
+Docker is primarily intended for production deployments or testing the production build locally.
+
+## Production Deployment
+
+For production deployments:
+
+1. Build the image:
+   ```bash
+   docker build -t stackrender:production .
+   ```
+
+2. Push to your container registry:
+   ```bash
+   docker tag stackrender:production your-registry/stackrender:latest
+   docker push your-registry/stackrender:latest
+   ```
+
+3. Deploy using your orchestration tool (Kubernetes, Docker Swarm, etc.)
+
+## Additional Resources
+
+- [Docker Documentation](https://docs.docker.com/)
+- [Docker Compose Documentation](https://docs.docker.com/compose/)
+- [Nginx Documentation](https://nginx.org/en/docs/)