Added --export command

[craigloewen-msft]

I made a PR to add the --export command to nerdctl.

I did it to solve issues like #1854 and since I saw it had support from the maintainers.

I also looked at this PR for inspiration: https://github.com/containerd/nerdctl/pull/2161/files

Please let me know if you want any changes or if it looks good!

[craigloewen-msft]

@AkihiroSuda is this change good to go? Anything else I can do to help merge this in? :)

[AkihiroSuda]

Not compilable 😞

https://github.com/containerd/nerdctl/actions/runs/16174086400/job/45654760202?pr=4405

# github.com/containerd/nerdctl/v2/pkg/cmd/container
Error: pkg/cmd/container/export.go:69:38: undefined: containerutil.MountSnapshotForContainer

[craigloewen-msft]

Not compilable 😞

https://github.com/containerd/nerdctl/actions/runs/16174086400/job/45654760202?pr=4405

# github.com/containerd/nerdctl/v2/pkg/cmd/container
Error: pkg/cmd/container/export.go:69:38: undefined: containerutil.MountSnapshotForContainer

Fixed the build!

[craigloewen-msft]

@AkihiroSuda I fixed the PR!
Looks like all the linting succeeds now, it's using the correct WriteDiff function and the only failing tests seem to be from other causes.

[apostasie]

@craigloewen-msft
TestExportRunningContainer is failing.
https://github.com/containerd/nerdctl/actions/runs/16451175448/job/46496554366?pr=4405#step:9:42466

[craigloewen-msft]

@apostasie thank you for the spot. Seems like somehow it's failing on Windows? Would need to dig into that further but for now I have to pause for a little to prioritize my other work.

[craigloewen-msft]

Errors seem to be an access denied error and a can't access the file since it's already in use. Maybe my use of the snapshotter here is bad??