Changing Hello World container name. (#130)

Making its name consistent with other examples (mnist). Renaming from `mlperf/mlbox:hello_world` to `mlperf/mlbox_hello_world:0.0.1`.

Author: sergey-serebryakov

examples/hello_world/platforms/docker.yaml

@@ -5,4 +5,4 @@ platform:
   name: "docker"
   version: ">=18.01"
 container:
-  image: "mlperf/mlbox:hello_world"
+  image: "mlperf/mlbox_hello_world:0.0.1"