jenkins的代理设置，方便下载插件

jenkins在下载插件的时候，总是网络不通，需要设置代理跨越长城

java.net.SocketTimeoutException: connect timed out

Caused: java.net.SocketTimeoutException: connect timed out

Caused: java.io.IOException: Failed to load http://updates.jenkins-ci.org/download/plugins/docker-commons/1.8/docker-commons.hpi to /var/lib/jenkins/plugins/docker-commons.jpi.tmp
	at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:1146)

Caused: hudson.util.IOException2: Failed to download from http://updates.jenkins-ci.org/download/plugins/docker-commons/1.8/docker-commons.hpi

设置方法：