Support podman for building release images

podman returns nozero exit code for `docker buildx`, because
it misses a subcommand.

`docker buildx version` should work both in podman and docker. Tested both
with docker-ce-20.10.18 + docker-buildx-plugin-0.10.2 and podman-4.5.0 +
podman-docker.

hack/lib/util.sh

@@ -729,7 +729,8 @@ function kube::util::ensure-cfssl {
 # Check if we have "docker buildx" commands available
 #
 function kube::util::ensure-docker-buildx {
-  if docker buildx >/dev/null 2>&1; then
+  # podman returns 0 on `docker buildx version`, docker on `docker buildx`. One of them must succeed.
+  if docker buildx version >/dev/null 2>&1 || docker buildx >/dev/null 2>&1; then
     return 0
   else
     echo "ERROR: docker buildx not available. Docker 19.03 or higher is required with experimental features enabled"