update Java version in dockercompose file

setup/docker-compose.yml

@@ -27,7 +27,7 @@ services:
       - local2
 
   server1:
-    image: java:8
+    image: openjdk:12
     volumes:
       - ../bookstore/build/libs:/code
     command: "java -jar bookstore.jar 1"
@@ -38,7 +38,7 @@ services:
       - local1
 
   server2:
-    image: java:8
+    image: openjdk:12
     volumes:
       - ../bookstore/build/libs:/code
     command: "java -jar bookstore.jar 2"