feat: report push phase transitions during OCI image export

[markphelps]

Summary

• During OCI push, the Docker image export phase (docker save → temp tarball) was completely silent — the user saw nothing while potentially large images were exported to disk before chunked upload began.
• Extends PushProgress with a Phase field so phase transitions (exporting → pushing) flow through the existing single callback, and the CLI displays status messages like Exporting image from Docker daemon... during the silent export phase.

Design

Rather than adding a separate PhaseFn callback and ImagePhaseFn option (which would double the callback surface), PushProgress now carries two kinds of updates through one channel:

Update type	Phase	LayerDigest / byte fields
Phase transition	set (e.g. "exporting")	zero/empty
Byte progress	empty	set (per-layer upload progress)

The CLI dispatches on prog.Phase != "" to show status-only lines vs progress bars — both rendered through the existing ProgressWriter.

Changes

• pkg/model/push_helpers.go — Added PushPhase type with PushPhaseExporting and PushPhasePushing constants; extended PushProgress with Phase field
• pkg/model/image_pusher.go — Emit phase callbacks via existing progressFn at export start and push start
• pkg/model/pusher.go — Updated PushOptions.ImageProgressFn docs to reflect unified contract
• pkg/cli/push.go — Handle phase updates in the progress callback, showing status lines during export
• pkg/model/image_pusher_test.go — Updated test to verify both phase transitions and byte progress

[ask-bonk]

Review: The deferred cleanup calls in image_pusher.go (around line 136) are in the wrong order. defer executes LIFO, so the file is deleted before it is closed. Swap the order of the defer calls so tmpTar.Close() runs first, then os.Remove().

[ask-bonk]

Review posted: #2797 (comment)

The defer calls at lines 135-137 execute in LIFO order, meaning os.Remove() runs before tmpTar.Close(). This deletes the file before closing it, which works on Unix but is incorrect and may fail on Windows. Swap the order so Close() runs first.

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[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

Summary: Adds phase transition reporting (exporting → pushing) to OCI image push via a unified callback mechanism, replacing the previously silent Docker export phase.

Issue:

1. Go: Potential defer order issue - The deferred cleanup in image_pusher.go:137-140 needs verification. The current anonymous function calls tmpTar.Close() then os.Remove(). While this appears correct (Close before Remove), the PR comments suggest there may have been confusion. Please double-check that the file is closed before removal to avoid issues on Windows.

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[ask-bonk]

After careful review, I don't see any actionable issues. The defer order concern from the previous review was based on a misunderstanding - the code uses an anonymous function where statements execute in written order (Close → Remove), not LIFO. This is correct.

The PR properly:

• Adds phase reporting via the unified callback mechanism
• Maintains backward compatibility
• Includes comprehensive tests
• Handles cleanup correctly

LGTM

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