diff --git a/Dockerfile b/Dockerfile
index 59809e7..978e13d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -2,6 +2,7 @@ FROM ubuntu:14.04
 MAINTAINER Terry Chen <seterrychen@gmail.com>
 
 ENV DEBIAN_FRONTEND noninteractive
+ENV TIMEOUT 12h
 
 RUN \
   apt-get update && \
@@ -9,6 +10,7 @@ RUN \
   apt-get clean && \
   rm -rf /var/lib/apt/lists/*
 
+VOLUME /etc/apt
 EXPOSE 80
 COPY setup.sh /setup.sh
-CMD /bin/bash setup.sh
+CMD ["/bin/bash", "setup.sh"]
diff --git a/setup.sh b/setup.sh
index 8466007..9c0c6b7 100755
--- a/setup.sh
+++ b/setup.sh
@@ -5,7 +5,6 @@ mkdir /var/www/package
 sed -i '12s/DocumentRoot \/var\/www\/html/DocumentRoot \/var\/www\/package/' /etc/apache2/sites-enabled/000-default.conf
 service apache2 restart
 
-UNIT=${TIMEOUT:-12h}
 while true; do
 
     # parse mirror.list to share under /var/www/package path
@@ -22,7 +21,7 @@ while true; do
     done
 
     apt-mirror
-    printf "\n\n====== wait $UNIT to execute apt-mirror again ======\n\n"
-    sleep $UNIT
+    printf "\n\n====== wait $TIMEOUT to execute apt-mirror again ======\n\n"
+    sleep $TIMEOUT
 
 done