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/golang/protobuf/proto/pointer_reflect.go

@@ -79,10 +79,13 @@ func toPointer(i *Message) pointer {
 
 // toAddrPointer converts an interface to a pointer that points to
 // the interface data.
-func toAddrPointer(i *interface{}, isptr bool) pointer {
+func toAddrPointer(i *interface{}, isptr, deref bool) pointer {
 	v := reflect.ValueOf(*i)
 	u := reflect.New(v.Type())
 	u.Elem().Set(v)
+	if deref {
+		u = u.Elem()
+	}
 	return pointer{v: u}
 }