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>

build/dial.go

@@ -0,0 +1,62 @@
+package build
+
+import (
+	"context"
+	stderrors "errors"
+	"net"
+
+	"github.com/containerd/containerd/platforms"
+	"github.com/docker/buildx/builder"
+	"github.com/docker/buildx/util/progress"
+	v1 "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+)
+
+func Dial(ctx context.Context, nodes []builder.Node, pw progress.Writer, platform *v1.Platform) (net.Conn, error) {
+	nodes, err := filterAvailableNodes(nodes)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(nodes) == 0 {
+		return nil, errors.New("no nodes available")
+	}
+
+	var pls []v1.Platform
+	if platform != nil {
+		pls = []v1.Platform{*platform}
+	}
+
+	opts := map[string]Options{"default": {Platforms: pls}}
+	resolved, err := resolveDrivers(ctx, nodes, opts, pw)
+	if err != nil {
+		return nil, err
+	}
+
+	var dialError error
+	for _, ls := range resolved {
+		for _, rn := range ls {
+			if platform != nil {
+				p := *platform
+				var found bool
+				for _, pp := range rn.platforms {
+					if platforms.Only(p).Match(pp) {
+						found = true
+						break
+					}
+				}
+				if !found {
+					continue
+				}
+			}
+
+			conn, err := nodes[rn.driverIndex].Driver.Dial(ctx)
+			if err == nil {
+				return conn, nil
+			}
+			dialError = stderrors.Join(err)
+		}
+	}
+
+	return nil, errors.Wrap(dialError, "no nodes available")
+}