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/k8s.io/client-go/util/exec/exec.go

@@ -0,0 +1,52 @@
+/*
+Copyright 2014 The Kubernetes Authors.
+
+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 exec
+
+// ExitError is an interface that presents an API similar to os.ProcessState, which is
+// what ExitError from os/exec is.  This is designed to make testing a bit easier and
+// probably loses some of the cross-platform properties of the underlying library.
+type ExitError interface {
+	String() string
+	Error() string
+	Exited() bool
+	ExitStatus() int
+}
+
+// CodeExitError is an implementation of ExitError consisting of an error object
+// and an exit code (the upper bits of os.exec.ExitStatus).
+type CodeExitError struct {
+	Err  error
+	Code int
+}
+
+var _ ExitError = CodeExitError{}
+
+func (e CodeExitError) Error() string {
+	return e.Err.Error()
+}
+
+func (e CodeExitError) String() string {
+	return e.Err.Error()
+}
+
+func (e CodeExitError) Exited() bool {
+	return true
+}
+
+func (e CodeExitError) ExitStatus() int {
+	return e.Code
+}