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_DEPLOYMENT.md

@@ -26,6 +26,8 @@ MinIO console: `http://localhost:9001` (minioadmin / minioadmin)
 
 ## Production Deployment
 
+## Production Deployment
+
 ### 1. Prepare Environment
 
 Copy and configure the production environment file:
@@ -34,7 +36,13 @@ Copy and configure the production environment file:
 cp docker/.env.production .env.production
 ```
 
-Edit `.env.production` and set:
+Or for Portainer deployment, copy the stack environment file:
+
+```bash
+cp docker/stack.env stack.env.local
+```
+
+Edit the file and set:
 - `POSTGRES_PASSWORD` - Strong random password
 - `REDIS_PASSWORD` - Strong random password
 - `MINIO_ROOT_PASSWORD` - Strong random password
@@ -44,6 +52,7 @@ Edit `.env.production` and set:
 - `GOOGLE_CLIENT_ID` / `GOOGLE_CLIENT_SECRET` - OAuth credentials
 - `OPENROUTER_API_KEY` - AI provider key
 - `SMTP_*` - Email configuration
+- `NEXT_PUBLIC_VAPID_PUBLIC_KEY` / `VAPID_PRIVATE_KEY` - Push notification keys
 
 ### 2. Build Docker Image
 
@@ -59,7 +68,67 @@ Or let Docker Compose build it automatically.
 docker compose -f docker/docker-compose.deploy.yml --env-file .env.production up -d
 ```
 
-### 4. Verify Deployment
+## Portainer Deployment
+
+Portainer provides a web UI for managing Docker containers and deploying stacks easily.
+
+### Portainer Setup
+
+1. **Prepare Environment File**
+
+   Copy the stack environment template:
+   ```bash
+   cp docker/stack.env stack.env.local
+   ```
+
+   Edit `stack.env.local` with your production values.
+
+2. **Access Portainer**
+
+   Open your Portainer instance (typically `https://portainer.your-domain.com`).
+
+3. **Deploy Stack**
+
+   - Go to **Stacks** → **Add Stack**
+   - Choose **Docker Compose** mode
+   - Select **Upload** to upload the compose file, or use **Editor** to paste the contents of `docker/docker-compose.deploy.yml`
+   - Copy the contents of `docker/stack.env` into the **Environment variables** section
+   - Click **Deploy the stack**
+
+   **Alternative: Using Environment File UI**
+   - In the stack creation form, there's an "Environment file" option
+   - You can paste the entire `stack.env` content there
+   - Portainer will parse and use all variables
+
+4. **Verify Deployment**
+
+   - Check the **Containers** tab to see all services running
+   - View logs by clicking on each container
+   - Verify the app is accessible at `https://your-domain.com`
+
+### Portainer Stack Update
+
+To update the application after code changes:
+
+1. Build new Docker image locally:
+   ```bash
+   docker build -f Dockerfile -t epicure-web:latest .
+   docker push your-registry/epicure-web:latest
+   ```
+
+2. In Portainer:
+   - Go to your stack
+   - Click **Editor**
+   - Update the image reference if needed
+   - Click **Update the stack**
+
+### Portainer Secrets (Optional)
+
+For better security, use Portainer Secrets instead of environment variables:
+
+1. Create a secret in Portainer with sensitive values
+2. Reference it in the compose file: `${POSTGRES_PASSWORD}`
+3. Portainer will inject the secret value during deployment
 
 ```bash
 # Check all services are running