cli/command/completion: add FromList utility

It's an alias for cobra.FixedCompletions but takes a variadic list
of strings, so that it's not needed to construct an array for this.

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

cmd/docker/completions.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"github.com/docker/cli/cli/command/completion"
 	"github.com/docker/cli/cli/context/store"
 	"github.com/spf13/cobra"
 )
@@ -19,13 +20,7 @@ func registerCompletionFuncForGlobalFlags(contextStore store.Store, cmd *cobra.C
 	if err != nil {
 		return err
 	}
-	err = cmd.RegisterFlagCompletionFunc(
-		"log-level",
-		func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
-			values := []string{"debug", "info", "warn", "error", "fatal"}
-			return values, cobra.ShellCompDirectiveNoFileComp
-		},
-	)
+	err = cmd.RegisterFlagCompletionFunc("log-level", completion.FromList("debug", "info", "warn", "error", "fatal"))
 	if err != nil {
 		return err
 	}