client: add reset option for local cache exporter

[jirimoravcik]

Add reset=true attribute to the local cache exporter that removes unreferenced blobs from the cache directory after export, preventing unbounded growth.

The logic is very simple:

1. Go through the index.json to get all the referenced manifests
2. Create a set of referenced blobs (check each referenced manifest)
3. Walk all the blobs in the cache directory, if there's a blob that is not referenced by any manifest, flag it for deletion
4. Delete all blobs flagged for deletion

I think cleanup could also work as the attribute name.

Resolves #1896

[Copilot]

Pull request overview

Adds a reset=true option to the local cache exporter so exported cache directories can be cleaned up (unreferenced blobs removed) after export, preventing unbounded growth. This fits into the client-side cache export flow that already updates the local index.json after a solve.

Changes:

• Parse a new reset attribute for type=local cache exports and trigger a post-export cleanup.
• Implement resetCacheStore to compute referenced blobs from index.json and delete unreferenced blobs.
• Add unit + integration tests and document the new option in README.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
client/solve.go	Parses reset attr, tracks stores to reset, and implements blob cleanup after export.
client/solve_resetcache_test.go	Unit tests for resetCacheStore behavior (currently image-manifest oriented).
client/client_test.go	Integration test for reset=true behavior with local cache export.
README.md	Documents the new reset=<true|false> option for local cache exporter.

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[jirimoravcik]

Fixed the lint, other errors seem to be 50x responses from DockerHub API.