Add .env.example file and update docker-compose configurations for environment variables

docker-compose.portainer.dev.yml

@@ -14,11 +14,11 @@ services:
       - APP_DEBUG=false
       - APP_URL=https://dev-admin.merchbay.app
       - DB_CONNECTION=mysql
-      - DB_HOST=your-mysql-host
-      - DB_PORT=3306
-      - DB_DATABASE=merchbay_admin_dev
-      - DB_USERNAME=your-mysql-user
-      - DB_PASSWORD=your-mysql-password
+      - DB_HOST=${DB_HOST}
+      - DB_PORT=${DB_PORT:-3306}
+      - DB_DATABASE=${DB_DATABASE}
+      - DB_USERNAME=${DB_USERNAME}
+      - DB_PASSWORD=${DB_PASSWORD}
     volumes:
       - app_storage_dev:/var/www/html/storage
       - app_uploads_dev:/var/www/html/public/uploads