move JENKINS_VERSION later in Dockerfile to share more layers
diff --git a/Dockerfile b/Dockerfile
index ed7ec74..2330f73 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -2,7 +2,6 @@
 
 RUN apt-get update && apt-get install -y wget git curl zip && rm -rf /var/lib/apt/lists/*
 
-ENV JENKINS_VERSION 1.565.3
 ENV JENKINS_HOME /var/jenkins_home
 
 # Jenkins is ran with user `jenkins`, uid = 1000
@@ -22,6 +21,8 @@
 
 COPY init.groovy /usr/share/jenkins/ref/init.groovy.d/tcp-slave-angent-port.groovy
 
+ENV JENKINS_VERSION 1.565.3
+
 # could use ADD but this one does not check Last-Modified header 
 # see https://github.com/docker/docker/issues/8331
 RUN curl -L http://mirrors.jenkins-ci.org/war-stable/$JENKINS_VERSION/jenkins.war -o /usr/share/jenkins/jenkins.war