Add dial-stdio command

This allows the buildx CLI to act a proxy to the configured instance.
It allows external code to use buildx itself as a driver for connecting
to buildkitd instances.

Instance and node selection should follow the same semantics as as
`buildx build`, including taking into account the `BUILDX_BUILDER` env
var and the `--builder` global flag.

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

commands/root.go

@@ -83,6 +83,7 @@ func addCommands(cmd *cobra.Command, dockerCli command.Cli) {
 		buildCmd(dockerCli, opts, nil),
 		bakeCmd(dockerCli, opts),
 		createCmd(dockerCli),
+		dialStdioCmd(dockerCli, opts),
 		rmCmd(dockerCli, opts),
 		lsCmd(dockerCli),
 		useCmd(dockerCli, opts),