[mc] Adjust mem with MEMORY and INIT_/MAX_MEMORY

For #126

minecraft-server/Dockerfile

@@ -40,6 +40,6 @@ ENTRYPOINT [ "/start" ]
 
 ENV UID=1000 GID=1000 \
     MOTD="A Minecraft Server Powered by Docker" \
-    JVM_XX_OPTS="-XX:+UseG1GC" MAX_MEMORY="1G" \
+    JVM_XX_OPTS="-XX:+UseG1GC" MEMORY="1G" \
     TYPE=VANILLA VERSION=LATEST FORGEVERSION=RECOMMENDED LEVEL=world PVP=true DIFFICULTY=easy \
     LEVEL_TYPE=DEFAULT GENERATOR_SETTINGS= WORLD= MODPACK= ONLINE_MODE=TRUE CONSOLE=true