Update test Dockerfiles to use PARENT_IMAGE build arg

Author: kabicin

test/test-pet-clinic/Dockerfile

@@ -1,6 +1,6 @@
 
-ARG IMAGE=ibmcom/websphere-liberty:kernel-java8-openj9-ubi
-FROM ${IMAGE} as staging
+ARG PARENT_IMAGE=ibmcom/websphere-liberty:kernel-java8-openj9-ubi
+FROM $PARENT_IMAGE as staging
 
 COPY --chown=1001:0 server.xml /config
 COPY --chown=1001:0 spring-petclinic-2.1.0.BUILD-SNAPSHOT.jar /staging/myFatApp.jar
@@ -13,7 +13,7 @@ RUN features.sh \
 
 RUN configure.sh
 
-FROM ${IMAGE}
+FROM $PARENT_IMAGE
 
 COPY --from=staging /config/server.xml /config/server.xml
 

test/test-stock-quote/Dockerfile

@@ -13,7 +13,8 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
-FROM ibmcom/websphere-liberty:kernel-java17-openj9-ubi
+ARG PARENT_IMAGE=ibmcom/websphere-liberty:kernel-java17-openj9-ubi
+FROM $PARENT_IMAGE
 
 COPY --chown=1001:0 config /config/
 

test/test-stock-trader/Dockerfile

@@ -12,7 +12,8 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
-FROM ibmcom/websphere-liberty:kernel-java8-openj9-ubi
+ARG PARENT_IMAGE=ibmcom/websphere-liberty:kernel-java8-openj9-ubi
+FROM $PARENT_IMAGE
 
 COPY --chown=1001:0 config /config/