Add command to convert docker image to tar file

Add commands for
- tar to docker image
- docker image to tar

The repository is a list of simple docker commands. Look for existing command entries to understand the format. Research Docker CLI commands for saving an image to a tar file (`docker save`) and loading an image from a tar file (`docker load`). Add these commands with clear examples to the appropriate documentation file, likely a README or a markdown file in the repo. Verify by checking the repository structure for where commands are listed.

索引モデルが issue の本文から書いたものです。