builder-next: support DOCKER_RAMDISK

For https://github.com/kubernetes/minikube/issues/4143

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit b4247b433eec9f568dcfa3941af67abbd7ae8bf3)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

builder/builder-next/executor_unix.go

@@ -31,6 +31,7 @@ func newExecutor(root, cgroupParent string, net libnetwork.NetworkController, ro
 		CommandCandidates:   []string{"runc"},
 		DefaultCgroupParent: cgroupParent,
 		Rootless:            rootless,
+		NoPivot:             os.Getenv("DOCKER_RAMDISK") != "",
 	}, networkProviders)
 }