[Bug]: `container export` should export a tar file.

[jglogan]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

• The functionality of container export is unexpected. It should be the inverse operation to container import; that is, it should produce an image tar file for a container.
• The subcommands for command groups in container --help should be in alphabetical order. Right now, export appears as the last entry in the group.
• The command reference documentation needs to include material for container export.

Repro:

container builder start
container stop buildkit
container export buildkit

Current behavior

• no image tar file is produced
• a new buildkit image exists

Expected behavior

container export should have similar options to and should function similarly to docker export, and should produce an image tar file.

Environment

- OS: 26
- Xcode: 26
- Container: 0.9.0

Relevant log output

N/A

Code of Conduct

• I agree to follow this project's Code of Conduct