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/docker-compose.deploy.yml

@@ -68,6 +68,10 @@ services:
     build:
       context: ..
       dockerfile: Dockerfile
+      args:
+        NEXT_PUBLIC_VAPID_PUBLIC_KEY: ${NEXT_PUBLIC_VAPID_PUBLIC_KEY}
+        BETTER_AUTH_SECRET: ${BETTER_AUTH_SECRET}
+        BETTER_AUTH_URL: ${BETTER_AUTH_URL}
     restart: always
     environment:
       DATABASE_URL: postgresql://${POSTGRES_USER:-epicure}:${POSTGRES_PASSWORD:-changeme}@postgres:5432/${POSTGRES_DB:-epicure}