cli: add completion to list builder names

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

util/cobrautil/completion/completion.go

@@ -4,6 +4,9 @@ import (
 	"strings"
 
 	"github.com/docker/buildx/bake"
+	"github.com/docker/buildx/builder"
+	"github.com/docker/buildx/store/storeutil"
+	"github.com/docker/cli/cli/command"
 	"github.com/spf13/cobra"
 )
 
@@ -36,3 +39,24 @@ func BakeTargets(files []string) ValidArgsFn {
 		return filtered, cobra.ShellCompDirectiveNoFileComp
 	}
 }
+
+func BuilderNames(dockerCli command.Cli) ValidArgsFn {
+	return func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		txn, release, err := storeutil.GetStore(dockerCli)
+		if err != nil {
+			return nil, cobra.ShellCompDirectiveError
+		}
+		defer release()
+		builders, err := builder.GetBuilders(dockerCli, txn)
+		if err != nil {
+			return nil, cobra.ShellCompDirectiveError
+		}
+		var filtered []string
+		for _, b := range builders {
+			if toComplete == "" || strings.HasPrefix(b.Name, toComplete) {
+				filtered = append(filtered, b.Name)
+			}
+		}
+		return filtered, cobra.ShellCompDirectiveNoFileComp
+	}
+}