Add builder as a global flag.

This allows all subcommands to use this flag.
Additionally reads the default value for the flag from the
`BUILDX_BUILDER` env var.

Precedence is:

CLI ARG > flag > env var > config file

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

commands/root.go

@@ -1,10 +1,13 @@
 package commands
 
 import (
+	"os"
+
 	imagetoolscmd "github.com/docker/buildx/commands/imagetools"
 	"github.com/docker/cli/cli-plugins/plugin"
 	"github.com/docker/cli/cli/command"
 	"github.com/spf13/cobra"
+	"github.com/spf13/pflag"
 )
 
 func NewRootCmd(name string, isPlugin bool, dockerCli command.Cli) *cobra.Command {
@@ -22,21 +25,32 @@ func NewRootCmd(name string, isPlugin bool, dockerCli command.Cli) *cobra.Comman
 	return cmd
 }
 
+type rootOptions struct {
+	builder string
+}
+
 func addCommands(cmd *cobra.Command, dockerCli command.Cli) {
+	opts := &rootOptions{}
+	rootFlags(opts, cmd.PersistentFlags())
+
 	cmd.AddCommand(
-		buildCmd(dockerCli),
-		bakeCmd(dockerCli),
+		buildCmd(dockerCli, opts),
+		bakeCmd(dockerCli, opts),
 		createCmd(dockerCli),
-		rmCmd(dockerCli),
+		rmCmd(dockerCli, opts),
 		lsCmd(dockerCli),
-		useCmd(dockerCli),
-		inspectCmd(dockerCli),
-		stopCmd(dockerCli),
+		useCmd(dockerCli, opts),
+		inspectCmd(dockerCli, opts),
+		stopCmd(dockerCli, opts),
 		installCmd(dockerCli),
 		uninstallCmd(dockerCli),
 		versionCmd(dockerCli),
-		pruneCmd(dockerCli),
-		duCmd(dockerCli),
+		pruneCmd(dockerCli, opts),
+		duCmd(dockerCli, opts),
 		imagetoolscmd.RootCmd(dockerCli),
 	)
 }
+
+func rootFlags(options *rootOptions, flags *pflag.FlagSet) {
+	flags.StringVar(&options.builder, "builder", os.Getenv("BUILDX_BUILDER"), "Override the configured builder instance")
+}