Implement tests for Dumplet

[kurama]

Description

Now that the core functionality is implemented, we need to add proper unit tests and integration tests to ensure everything works reliably.

Tasks:

• Add unit tests for error handling (e.g., when Docker is unavailable).

• Add integration tests if possible.

• Validate that the .tar file is correctly generated.

• Ensure that the container is correctly removed after execution.

Definition of Done:

• Running cargo test should validate that the code behaves correctly.
• The .tar file should be successfully created and contain a valid filesystem.
• Tests should not leave any artifacts (containers or files).