Better set JVM opts in previewer

The JVM heap size is currrently hard-coded in the previewer thru the $TOMCAT_HOME/bin/setenv.sh script.

export CATALINA_OPTS="$CATALINA_OPTS -Xms512m"
export CATALINA_OPTS="$CATALINA_OPTS -Xmx2048m"

If the container resource is limited to 2Gi, it will eventually raise an OOM. https://blog.softwaremill.com/docker-support-in-new-java-8-finally-fd595df0ca54

We should dynamically set the size of the heap to 70% of the container.