Update beta Dockerfile to use PARENT_IMAGE

Author: kabicin

beta/Dockerfile

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM ibmjava:8-jre
+ARG PARENT_IMAGE=ibmjava:8-jre
+FROM $PARENT_IMAGE
 
 LABEL org.opencontainers.image.authors="Leo Christy Jesuraj, Arthur De Magalhaes, Chris Potter" \
       org.opencontainers.image.vendor="IBM" \