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/discovery/unstructured.go

@@ -0,0 +1,81 @@
+/*
+Copyright 2016 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 discovery
+
+import (
+	"reflect"
+
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+)
+
+// UnstructuredObjectTyper provides a runtime.ObjectTyper implementation for
+// runtime.Unstructured object based on discovery information.
+type UnstructuredObjectTyper struct {
+	typers []runtime.ObjectTyper
+}
+
+// NewUnstructuredObjectTyper returns a runtime.ObjectTyper for
+// unstructured objects based on discovery information. It accepts a list of fallback typers
+// for handling objects that are not runtime.Unstructured. It does not delegate the Recognizes
+// check, only ObjectKinds.
+// TODO this only works for the apiextensions server and doesn't recognize any types.  Move to point of use.
+func NewUnstructuredObjectTyper(typers ...runtime.ObjectTyper) *UnstructuredObjectTyper {
+	dot := &UnstructuredObjectTyper{
+		typers: typers,
+	}
+	return dot
+}
+
+// ObjectKinds returns a slice of one element with the group,version,kind of the
+// provided object, or an error if the object is not runtime.Unstructured or
+// has no group,version,kind information. unversionedType will always be false
+// because runtime.Unstructured object should always have group,version,kind
+// information set.
+func (d *UnstructuredObjectTyper) ObjectKinds(obj runtime.Object) (gvks []schema.GroupVersionKind, unversionedType bool, err error) {
+	if _, ok := obj.(runtime.Unstructured); ok {
+		gvk := obj.GetObjectKind().GroupVersionKind()
+		if len(gvk.Kind) == 0 {
+			return nil, false, runtime.NewMissingKindErr("object has no kind field ")
+		}
+		if len(gvk.Version) == 0 {
+			return nil, false, runtime.NewMissingVersionErr("object has no apiVersion field")
+		}
+		return []schema.GroupVersionKind{gvk}, false, nil
+	}
+	var lastErr error
+	for _, typer := range d.typers {
+		gvks, unversioned, err := typer.ObjectKinds(obj)
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		return gvks, unversioned, nil
+	}
+	if lastErr == nil {
+		lastErr = runtime.NewNotRegisteredErrForType(reflect.TypeOf(obj))
+	}
+	return nil, false, lastErr
+}
+
+// Recognizes returns true if the provided group,version,kind was in the
+// discovery information.
+func (d *UnstructuredObjectTyper) Recognizes(gvk schema.GroupVersionKind) bool {
+	return false
+}
+
+var _ runtime.ObjectTyper = &UnstructuredObjectTyper{}