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/namespace.go

@@ -0,0 +1,25 @@
+// Copyright 2012 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 appengine
+
+import (
+	"fmt"
+	"regexp"
+
+	"golang.org/x/net/context"
+
+	"google.golang.org/appengine/internal"
+)
+
+// Namespace returns a replacement context that operates within the given namespace.
+func Namespace(c context.Context, namespace string) (context.Context, error) {
+	if !validNamespace.MatchString(namespace) {
+		return nil, fmt.Errorf("appengine: namespace %q does not match /%s/", namespace, validNamespace)
+	}
+	return internal.NamespacedContext(c, namespace), nil
+}
+
+// validNamespace matches valid namespace names.
+var validNamespace = regexp.MustCompile(`^[0-9A-Za-z._-]{0,100}$`)