Add Portainer stack.env support for environment variable management

- Add docker/stack.env template for Portainer deployment
- Update Dockerfile to accept build args for Next.js environment variables
- Update docker-compose.deploy.yml to pass build args from environment
- Add comprehensive Portainer deployment guide in DOCKER_DEPLOYMENT.md
- Document environment variable setup for both CLI and Portainer deployments

docker/stack.env

@@ -0,0 +1,68 @@
+# Portainer Stack Environment Variables
+# Copy this file to stack.env.local and fill in your production values
+# Portainer will read this file when deploying the stack
+
+# ============================================
+# Database Configuration
+# ============================================
+POSTGRES_DB=epicure
+POSTGRES_USER=epicure
+POSTGRES_PASSWORD=changeme_strong_password_here
+DB_HOST=postgres
+
+# ============================================
+# Redis Configuration
+# ============================================
+REDIS_PASSWORD=changeme_strong_password_here
+
+# ============================================
+# MinIO / S3 Storage Configuration
+# ============================================
+MINIO_ROOT_USER=minioadmin
+MINIO_ROOT_PASSWORD=changeme_strong_password_here
+
+# ============================================
+# Better Auth Configuration
+# ============================================
+BETTER_AUTH_SECRET=changeme_generate_with_openssl_rand_base64_32
+BETTER_AUTH_URL=https://your-domain.com
+
+# ============================================
+# Domain Configuration
+# ============================================
+DOMAIN=your-domain.com
+
+# ============================================
+# OAuth Configuration
+# ============================================
+GOOGLE_CLIENT_ID=your-google-client-id
+GOOGLE_CLIENT_SECRET=your-google-client-secret
+
+# ============================================
+# AI Provider Configuration
+# ============================================
+OPENROUTER_API_KEY=your-openrouter-api-key
+OPENROUTER_DEFAULT_MODEL=nvidia/nemotron-3-ultra-550b-a55b:free
+OPENAI_API_KEY=
+ANTHROPIC_API_KEY=
+
+# ============================================
+# Email / SMTP Configuration
+# ============================================
+SMTP_HOST=smtp.eu.mailgun.org
+SMTP_PORT=587
+SMTP_SECURE=false
+SMTP_USER=your-email@example.com
+SMTP_PASS=your-smtp-password
+SMTP_FROM=Epicure <noreply@example.com>
+
+# ============================================
+# Push Notifications
+# ============================================
+NEXT_PUBLIC_VAPID_PUBLIC_KEY=your-vapid-public-key
+VAPID_PRIVATE_KEY=your-vapid-private-key
+
+# ============================================
+# Node Environment
+# ============================================
+NODE_ENV=production