fix: add curl and pass environment variables to Docker build

- Install curl in Alpine image for health checks
- Add ARG and ENV declarations for NEXT_PUBLIC_* variables in builder stage
- Pass build args from docker-compose.yml to Dockerfile
- Update health checks to use curl instead of node -e
- Update .env.example with helpful comments for Docker vs local dev

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -1,6 +1,8 @@
 # Pastel API Configuration
 # URL of the Pastel API instance
-NEXT_PUBLIC_API_URL=http://localhost:3000
+# For Docker Compose: use http://pastel-api:3001
+# For local development: use http://localhost:3001
+NEXT_PUBLIC_API_URL=http://localhost:3001
 
 # Application URL (used for sharing and OG images)
 NEXT_PUBLIC_APP_URL=http://localhost:3000