cli/command/completion: change Platforms to return a cobra.CompletionFunc

It's adding a slight indirection by constructing a function when called,
but makes the completion functions more consistent, the signature easier
to read, and making the return type a [cobra.CompletionFunc] makes it
more transparent what it's intended for, and helps discovery of functions
that provide completion.

[cobra.CompletionFunc]: https://pkg.go.dev/github.com/spf13/cobra#CompletionFunc

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/history.go

@@ -52,7 +52,7 @@ func newHistoryCommand(dockerCLI command.Cli) *cobra.Command {
 	flags.StringVar(&opts.platform, "platform", "", `Show history for the given platform. Formatted as "os[/arch[/variant]]" (e.g., "linux/amd64")`)
 	_ = flags.SetAnnotation("platform", "version", []string{"1.48"})
 
-	_ = cmd.RegisterFlagCompletionFunc("platform", completion.Platforms)
+	_ = cmd.RegisterFlagCompletionFunc("platform", completion.Platforms())
 	return cmd
 }