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/apimachinery/pkg/apis/meta/v1beta1/deepcopy.go

@@ -0,0 +1,44 @@
+/*
+Copyright 2017 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 v1beta1
+
+import "k8s.io/apimachinery/pkg/runtime"
+
+func (in *TableRow) DeepCopy() *TableRow {
+	if in == nil {
+		return nil
+	}
+
+	out := new(TableRow)
+
+	if in.Cells != nil {
+		out.Cells = make([]interface{}, len(in.Cells))
+		for i := range in.Cells {
+			out.Cells[i] = runtime.DeepCopyJSONValue(in.Cells[i])
+		}
+	}
+
+	if in.Conditions != nil {
+		out.Conditions = make([]TableRowCondition, len(in.Conditions))
+		for i := range in.Conditions {
+			in.Conditions[i].DeepCopyInto(&out.Conditions[i])
+		}
+	}
+
+	in.Object.DeepCopyInto(&out.Object)
+	return out
+}