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>

driver/docker-container/driver.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/docker/buildx/driver"
+	"github.com/docker/buildx/driver/bkimage"
 	"github.com/docker/buildx/util/progress"
 	"github.com/docker/docker/api/types"
 	dockertypes "github.com/docker/docker/api/types"
@@ -22,8 +23,6 @@ import (
 	"github.com/pkg/errors"
 )
 
-var defaultBuildkitImage = "moby/buildkit:buildx-stable-1" // TODO: make this verified
-
 type Driver struct {
 	driver.InitConfig
 	factory driver.Factory
@@ -54,7 +53,7 @@ func (d *Driver) Bootstrap(ctx context.Context, l progress.Logger) error {
 }
 
 func (d *Driver) create(ctx context.Context, l progress.SubLogger) error {
-	imageName := defaultBuildkitImage
+	imageName := bkimage.DefaultImage
 	if d.image != "" {
 		imageName = d.image
 	}