Add default admin credentials and improved initialization logging

DOCKER_INSTALLATION.md

@@ -37,9 +37,16 @@ That's it! 🎉
 - **Web Interface**: http://localhost:12345
 - **Update Server**: http://localhost:9000
 
-### Default Credentials
+### 🔑 Default Credentials
 
-On first run, create an admin account through the web interface.
+On first run, a default admin account is automatically created:
+
+```
+Username: admin
+Password: admin
+```
+
+**⚠️ IMPORTANT**: You will be prompted to change this password on first login!
 
 ## 🔧 Configuration Options