[mc] Use G1 GC by default and simplify max memory setting

For #126

minecraft-server/Dockerfile

@@ -38,6 +38,6 @@ ENTRYPOINT [ "/start" ]
 
 ENV UID=1000 GID=1000 \
     MOTD="A Minecraft Server Powered by Docker" \
-    JVM_OPTS="-Xmx1024M -Xms1024M" \
+    JVM_XX_OPTS="-XX:+UseG1GC" MAX_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