exporter: add local exporter mode=delete

[crazy-max]

partially fixes #2805

This PR adds support for a mode attribute on the local exporter output path.

The default behavior remains mode=copy, which preserves destination files that are not present in the exported result. A new mode=delete behavior is added so the destination matches the exported result by removing stale files and directories.

Because mode=delete can remove files under the destination directory, this feature is intentionally explicit opt-in and should be used only with trusted build definitions and carefully reviewed output destinations.

On client side like Buildx we should probably have some safety gate like --allow local-output-delete (or equivalent) for remote Bake invocations. Optionally block/require explicit allow when dest=. with mode=delete?

Maybe we could have some kind of scope attribute like mode=delete,delete-scope=dist,assets so it only deletes stale files under those relative paths (and reject unsafe scopes like .. or absolute paths)?

Also the backup mode from the issue is intentionally left for a follow-up change. I think we would need changes in fsutil.Receive (or diff callbacks).

[tonistiigi]

I'd call it delete like rsync --delete. In Buildx I think we need more restrictions. I'd only allow it for subdirs there, with a second-level opt-in needed to mess with the main workdir or things outside it.

[crazy-max]

I'd call it delete like rsync --delete. In Buildx I think we need more restrictions. I'd only allow it for subdirs there, with a second-level opt-in needed to mess with the main workdir or things outside it.

Agreed, renamed the mode from mirror to delete to align with rsync --delete semantics.