Prevent jenkins to leak zombie processes

fix issue #54

Dockerfile

@@ -18,6 +18,8 @@ VOLUME /var/jenkins_home
 # or config file with your custom jenkins Docker image.
 RUN mkdir -p /usr/share/jenkins/ref/init.groovy.d
 
+# Use tini as subreaper in Docker container to adopt zombie processes 
+RUN curl -fL https://github.com/krallin/tini/releases/download/v0.5.0/tini-static -o /bin/tini && chmod +x /bin/tini
 
 COPY init.groovy /usr/share/jenkins/ref/init.groovy.d/tcp-slave-agent-port.groovy
 
@@ -43,7 +45,7 @@ ENV COPY_REFERENCE_FILE_LOG $JENKINS_HOME/copy_reference_file.log
 USER jenkins
 
 COPY jenkins.sh /usr/local/bin/jenkins.sh
-ENTRYPOINT ["/usr/local/bin/jenkins.sh"]
+ENTRYPOINT ["/bin/tini", "--", "/usr/local/bin/jenkins.sh"]
 
 # from a derived Dockerfile, can use `RUN plugin.sh active.txt` to setup /usr/share/jenkins/ref/plugins from a support bundle
 COPY plugins.sh /usr/local/bin/plugins.sh