docker stats: assorted fixes and optimizations in rendering

[thaJeztah]

• relates to Optimise docker stats to not require clearing the whole screen #5625

cli/command/container: fix buffer reuse when printing stats

Don't write lines back into the same buffer that's being read from when
clearing lines; add a separate output buffer to construct the output,
then write it to the CLI's output at once (to prevent terminal flicker).

Relates to / introduced in cb2f95c.

cli/command/container: stats: add snapshot method

Move logic to capture a snapshot of the current stats to the stats struct.

cli/command/container: RunStats: rename buffer var for brevity

cli/command/container: RunStats: avoid bytes to strings conversions

This code is using a bytes.Buffer to render the stats, before writing
the results to the CLI's output. Let's try to use bytes where possible
instead of converting to a string;

• Use the buffer's Write (and Out().Write) to write directly to the
  buffer/writer where possible.
• Use io.WriteString instead of fmt.Printf
• Use bytes.SplitSeq instead of strings.SplitSeq

cli/command/container: statsFormatWrite: inline render func

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 23.80952% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/container/stats.go	0.00%	24 Missing ⚠️
cli/command/container/stats_helpers.go	0.00%	8 Missing ⚠️

📢 Thoughts on this report? Let us know!

[Copilot]

Pull request overview

This PR refactors and optimizes docker stats rendering by centralizing stats snapshot collection and reducing string conversions during output generation, aiming to improve performance and reduce terminal flicker.

Changes:

• Added a snapshot() method on the internal stats tracker to safely copy the tracked container list under lock.
• Updated RunStats to render using bytes.Buffer/io.WriteString and to avoid bytes -> string conversions.
• Simplified statsFormatWrite by inlining the per-entry render closure.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cli/command/container/stats.go	Switches rendering to byte-oriented writes and uses snapshot() to avoid holding the stats lock while building output.
cli/command/container/stats_helpers.go	Adds (*stats).snapshot() helper for point-in-time container list copying.
cli/command/container/formatter_stats.go	Minor refactor to inline the render function passed to ctx.Write.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[vvoland]

I think this is dead code since we already have the same check in L325?

[thaJeztah]

oh, you're right; I think it is; let me remove

[thaJeztah]

Removed 👍

There's more to cleanup in this code IMO; I hate the FormatContext having the buffer / writer attached here; it would've been much more organic if statsFormatWrite would just get a buffer passed in I think