Fix k8s container BASE_IMAGE (#2808)

docker/build_image.sh creates a CPU image with tag 'latest-cpu'.  Use
that tag when building the k8s kserve image.

Co-authored-by: Sean Bowman <r.bowman@striveworks.us>
Co-authored-by: lxning <23464292+lxning@users.noreply.github.com>

kubernetes/kserve/build_image.sh

@@ -2,7 +2,7 @@
 
 MACHINE=cpu
 DOCKER_TAG="pytorch/torchserve-kfs:latest"
-BASE_IMAGE="pytorch/torchserve:latest"
+BASE_IMAGE="pytorch/torchserve:latest-cpu"
 DOCKER_FILE="Dockerfile"
 BUILD_NIGHTLY=false
 USE_CUSTOM_TAG=false