[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

minecraft-server/README.md

@@ -608,10 +608,11 @@ By default, server checks connecting players against Minecraft's account databas
 
 ### Memory Limit
 
-The Java memory limit can be adjusted using the `JVM_OPTS` environment variable, where the default is
-the setting shown in the example (max and min at 1024 MB):
+By default the image declares a Java memory limit of 1 GB. That can be adjusted
+higher (or lower) by setting the `MAX_MEMORY` environment variable. For example,
+the following increases the memory limit to 8 GB:
 
-    docker run -e 'JVM_OPTS=-Xmx1024M -Xms1024M' ...
+    docker run -e MAX_MEMORY=8G ...
 
 ### /data ownership
 

minecraft-server/start-minecraft.sh

@@ -490,6 +490,11 @@ else
   EXTRA_ARGS=""
 fi
 
+if [[ ! -z $MAX_MEMORY ]]; then
+  # put prior JVM_OPTS at the end to give any memory settings there higher precedence
+  JVM_OPTS="-Xms${MAX_MEMORY} -Xmx${MAX_MEMORY} ${JVM_OPTS}"
+fi
+set -x
 if [[ ${TYPE} == "FEED-THE-BEAST" ]]; then
     echo "Running FTB server modpack start ..."
     exec sh ${FTB_SERVER_START}
@@ -497,8 +502,8 @@ else
     # If we have a bootstrap.txt file... feed that in to the server stdin
     if [ -f /data/bootstrap.txt ];
     then
-        exec java $JVM_OPTS -jar $SERVER "$@" $EXTRA_ARGS < /data/bootstrap.txt
+        exec java $JVM_XX_OPTS $JVM_OPTS -jar $SERVER "$@" $EXTRA_ARGS < /data/bootstrap.txt
     else
-        exec java $JVM_OPTS -jar $SERVER "$@" $EXTRA_ARGS
+        exec java $JVM_XX_OPTS $JVM_OPTS -jar $SERVER "$@" $EXTRA_ARGS
     fi
 fi