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/github.com/googleapis/gnostic/compiler/context.go

@@ -0,0 +1,43 @@
+// Copyright 2017 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package compiler
+
+// Context contains state of the compiler as it traverses a document.
+type Context struct {
+	Parent            *Context
+	Name              string
+	ExtensionHandlers *[]ExtensionHandler
+}
+
+// NewContextWithExtensions returns a new object representing the compiler state
+func NewContextWithExtensions(name string, parent *Context, extensionHandlers *[]ExtensionHandler) *Context {
+	return &Context{Name: name, Parent: parent, ExtensionHandlers: extensionHandlers}
+}
+
+// NewContext returns a new object representing the compiler state
+func NewContext(name string, parent *Context) *Context {
+	if parent != nil {
+		return &Context{Name: name, Parent: parent, ExtensionHandlers: parent.ExtensionHandlers}
+	}
+	return &Context{Name: name, Parent: parent, ExtensionHandlers: nil}
+}
+
+// Description returns a text description of the compiler state
+func (context *Context) Description() string {
+	if context.Parent != nil {
+		return context.Parent.Description() + "." + context.Name
+	}
+	return context.Name
+}