docs: add lab6 submission

[Aleliya]

Goal

Lab 6 has been completed on the basics of working with Docker: managing containers, creating images, working with networks and volumes for permanent data storage.

Changes

• The life cycle of containers, image exports, and dependencies between containers and images have been studied.
• Сustom Nginx image with custom HTML content has been created via docker commit
• Сustom Docker network with DNS resolution between containers has been set up
• Implemented permanent data storage using Docker volumes

Testing

• Clone the repository and checkout the feature/lab6 branch
• Navigate to labs/submission6.md

Checklist:

• Clear title and description
• Documentation/README updated if needed
• No secrets or large temporary files

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• Task 1 — Container Lifecycle & Image Management
• Task 2 — Custom Image Creation & Analysis
• Task 3 — Container Networking & Service Discovery
• Task 4 — Data Persistence with Volumes