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/main_vm.go

@@ -0,0 +1,48 @@
+// Copyright 2011 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.
+
+// +build !appengine
+
+package internal
+
+import (
+	"io"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+)
+
+func Main() {
+	installHealthChecker(http.DefaultServeMux)
+
+	port := "8080"
+	if s := os.Getenv("PORT"); s != "" {
+		port = s
+	}
+
+	host := ""
+	if IsDevAppServer() {
+		host = "127.0.0.1"
+	}
+	if err := http.ListenAndServe(host+":"+port, http.HandlerFunc(handleHTTP)); err != nil {
+		log.Fatalf("http.ListenAndServe: %v", err)
+	}
+}
+
+func installHealthChecker(mux *http.ServeMux) {
+	// If no health check handler has been installed by this point, add a trivial one.
+	const healthPath = "/_ah/health"
+	hreq := &http.Request{
+		Method: "GET",
+		URL: &url.URL{
+			Path: healthPath,
+		},
+	}
+	if _, pat := mux.Handler(hreq); pat != healthPath {
+		mux.HandleFunc(healthPath, func(w http.ResponseWriter, r *http.Request) {
+			io.WriteString(w, "ok")
+		})
+	}
+}