define JENKINS_OPTS so a derived image can customize jenkins launcher command line
defining some `ENV JENKINS_OPTS "..."`
Fix #21
diff --git a/jenkins.sh b/jenkins.sh
index 791f938..4bb7a77 100755
--- a/jenkins.sh
+++ b/jenkins.sh
@@ -1,4 +1,4 @@
 #! /bin/bash
 
-exec java $JAVA_OPTS -jar /usr/share/jenkins/jenkins.war --extractedFilesFolder=/var/lib/jenkins --prefix=$JENKINS_PREFIX $@
+exec java $JAVA_OPTS -jar /usr/share/jenkins/jenkins.war $JENKINS_OPTS --prefix=$JENKINS_PREFIX $@