new driver: kubernetes

Tested with `kind` and GKE.

Note: "nodes" shown in `docker buildx ls` are unrelated to Kubernetes "nodes".
Probably buildx should come up with an alternative term.

Usage:

  $ kind create cluster
  $ export KUBECONFIG="$(kind get kubeconfig-path --name="kind")"

  $ docker buildx create --driver kubernetes --driver-opt replicas=3 --use
  $ docker buildx build -t foo --load .

`--load` loads the image into the local Docker.

Driver opts:

  - `image=IMAGE` - Sets the container image to be used for running buildkit.
  - `namespace=NS` - Sets the Kubernetes namespace. Defaults to the current namespace.
  - `replicas=N` - Sets the number of `Pod` replicas. Defaults to 1.
  - `rootless=(true|false)` - Run the container as a non-root user without `securityContext.privileged`. Defaults to false.
  - `loadbalance=(sticky|random)` - Load-balancing strategy. If set to "sticky", the pod is chosen using the hash of the context path. Defaults to "sticky"

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/google.golang.org/appengine/internal/metadata.go

@@ -0,0 +1,61 @@
+// Copyright 2014 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package internal
+
+// This file has code for accessing metadata.
+//
+// References:
+//	https://cloud.google.com/compute/docs/metadata
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"net/url"
+)
+
+const (
+	metadataHost = "metadata"
+	metadataPath = "/computeMetadata/v1/"
+)
+
+var (
+	metadataRequestHeaders = http.Header{
+		"Metadata-Flavor": []string{"Google"},
+	}
+)
+
+// TODO(dsymonds): Do we need to support default values, like Python?
+func mustGetMetadata(key string) []byte {
+	b, err := getMetadata(key)
+	if err != nil {
+		log.Fatalf("Metadata fetch failed: %v", err)
+	}
+	return b
+}
+
+func getMetadata(key string) ([]byte, error) {
+	// TODO(dsymonds): May need to use url.Parse to support keys with query args.
+	req := &http.Request{
+		Method: "GET",
+		URL: &url.URL{
+			Scheme: "http",
+			Host:   metadataHost,
+			Path:   metadataPath + key,
+		},
+		Header: metadataRequestHeaders,
+		Host:   metadataHost,
+	}
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return nil, fmt.Errorf("metadata server returned HTTP %d", resp.StatusCode)
+	}
+	return ioutil.ReadAll(resp.Body)
+}