APT_CACHE_SERVER in Jenkinsfile points to a non reachable IP address

[rpannek]

In https://github.com/Pelagicore/softwarecontainer/blob/master/Jenkinsfile#L64t APT_CACHE_SERVER points to 10.8.36.16 which seems to be some kind of an internal IP adress which is not accessible from the Internet. It should either be removed or taken from the Jenkins ENV.