diff --git a/Dockerfile b/Dockerfile
index 3f52c78..3d8a86e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,24 +1,25 @@
 FROM ubuntu:14.04
 
-RUN echo "1.565.2" > .lts-version-number
+RUN apt-get update && apt-get install -y wget git curl zip && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends openjdk-7-jdk && rm -rf /var/lib/apt/lists/*
 
-RUN apt-get update && apt-get install -y wget git curl zip
-RUN apt-get update && apt-get install -y --no-install-recommends openjdk-7-jdk
-RUN wget -q -O - http://pkg.jenkins-ci.org/debian-stable/jenkins-ci.org.key | sudo apt-key add -
-RUN echo deb http://pkg.jenkins-ci.org/debian-stable binary/ >> /etc/apt/sources.list
-RUN apt-get update && apt-get install -y jenkins
-RUN apt-get clean
-RUN rm -rf /var/lib/apt/lists/*
-RUN usermod -m -d /var/jenkins_home jenkins
-RUN mkdir -p /var/jenkins_home && chown -R jenkins /var/jenkins_home
-ADD init.groovy /tmp/WEB-INF/init.groovy
-RUN cd /tmp && zip -g /usr/share/jenkins/jenkins.war WEB-INF/init.groovy && rm -rf /tmp/WEB-INF
-ADD ./jenkins.sh /usr/local/bin/jenkins.sh
-RUN chmod +x /usr/local/bin/jenkins.sh
-USER jenkins
+# gpg: key D50582E6: public key "Kohsuke Kawaguchi <kk@kohsuke.org>" imported
+# see also http://pkg.jenkins-ci.org/debian-stable/jenkins-ci.org.key
+RUN apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 150FDE3F7787E7D11EF4E12A9B7D32F2D50582E6
+# from: curl -sSL 'http://pkg.jenkins-ci.org/debian-stable/jenkins-ci.org.key' | docker run -i --rm debian bash -c 'gpg --import && gpg --fingerprint'
 
-VOLUME /var/jenkins_home
+RUN echo deb http://pkg.jenkins-ci.org/debian-stable binary/ > /etc/apt/sources.list.d/jenkins.list
+
+ENV JENKINS_VERSION 1.565.2
+
+RUN apt-get update && apt-get install -y jenkins="${JENKINS_VERSION}" && rm -rf /var/lib/apt/lists/*
+
 ENV JENKINS_HOME /var/jenkins_home
+RUN usermod -m -d "$JENKINS_HOME" jenkins && chown -R jenkins "$JENKINS_HOME"
+VOLUME /var/jenkins_home
+
+COPY init.groovy /tmp/WEB-INF/init.groovy
+RUN cd /tmp && zip -g /usr/share/jenkins/jenkins.war WEB-INF/init.groovy && rm -rf /tmp/WEB-INF
 
 # define url prefix for running jenkins behind Apache (https://wiki.jenkins-ci.org/display/JENKINS/Running+Jenkins+behind+Apache)
 ENV JENKINS_PREFIX /
@@ -29,4 +30,6 @@
 # will be used by attached slave agents:
 EXPOSE 50000
 
+COPY ./jenkins.sh /usr/local/bin/jenkins.sh
+USER jenkins
 CMD ["/usr/local/bin/jenkins.sh"]
diff --git a/jenkins.sh b/jenkins.sh
old mode 100644
new mode 100755