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>

docs/reference/buildx_dial-stdio.md

@@ -0,0 +1,47 @@
+# docker buildx dial-stdio
+
+<!---MARKER_GEN_START-->
+Proxy current stdio streams to builder instance
+
+### Options
+
+| Name         | Type     | Default | Description                                      |
+|:-------------|:---------|:--------|:-------------------------------------------------|
+| `--builder`  | `string` |         | Override the configured builder instance         |
+| `--platform` | `string` |         | Target platform: this is used for node selection |
+| `--progress` | `string` | `quiet` | Set type of progress output (auto, plain, tty).  |
+
+
+<!---MARKER_GEN_END-->
+
+## Description
+
+dial-stdio uses the stdin and stdout streams of the command to proxy to the configured builder instance.
+It is not intended to be used by humans, but rather by other tools that want to interact with the builder instance via BuildKit API.
+
+## Examples
+
+Example go program that uses the dial-stdio command wire up a buildkit client.
+This is for example use only and may not be suitable for production use.
+
+```go
+client.New(ctx, "", client.WithContextDialer(func(context.Context, string) (net.Conn, error) {
+    c1, c2 := net.Pipe()
+    cmd := exec.Command("docker", "buildx", "dial-stdio")
+    cmd.Stdin = c1
+    cmd.Stdout = c1
+
+    if err := cmd.Start(); err != nil {
+        c1.Close()
+        c2.Close()
+        return nil, err
+    }
+
+    go func() {
+        cmd.Wait()
+        c2.Close()
+    }()
+
+    return c2
+}))
+```