add configurable timeout for image transfer to gateway containerd

[tmckayus]

Summary

The transfer of very large sandbox images to containerd can timeout depending on the size of the image and the speed of the local host.

Changes

• crates/openshell-bootstrap/src/docker.rs — add connect_local_for_large_transfers() helper plus a DEFAULT_LARGE_TRANSFER_TIMEOUT_SECS constant; reads OPENSHELL_DOCKER_TIMEOUT_SECS so the timeout can be tuned per host/image without a rebuild. Default is 3600s.
• crates/openshell-bootstrap/src/build.rs — route the sandbox create --from image sideload through the new helper so docker save streams don't trip bollard's 120 s default.
• crates/openshell-bootstrap/src/lib.rs — same helper for the OPENSHELL_PUSH_IMAGES component-deploy path, for consistency.

Testing

Tested with large and small image files manually to verify configurable timeout is effective with no regression
Did not include any unit test.

[github-actions]

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[drew]

This PR looks good to me. Just some issues with code formatting and need to sign DCO. As a FYI we'll be moving off this k3s approach and use docker more natively on the host which should additionally resolve the issue.

Ticket for this work is here: #873. Hoping to land this by the end of the week.

[tmckayus]

I have read the DCO document and I hereby sign the DCO.