pkg/compose: implement Export using atomicwriter

[thaJeztah]

Replaces the use of cli/command.CopyToFile with an atomicwriter, as cli/command.CopyToFile will be deprecated in the next release.

What I did

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[thaJeztah]

FWIW; atomicwriter.New() also performs this validation, so we could have a look if we still need this check before we run that code 🤔

I should also have a look at; the copy commands;

compose/pkg/compose/cp.go

Lines 174 to 177 in a1f673d

	// Validate the destination path
	if err := command.ValidateOutputPathFileMode(dstStat.Mode); err != nil {
	return fmt.Errorf(`destination "%s:%s" must be a directory or a regular file: %w`, containerID, dstPath, err)
	}

compose/pkg/compose/cp.go

Lines 255 to 257 in a1f673d

	if err := command.ValidateOutputPath(dstPath); err != nil {
	return err
	}