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.

[frznlogic]

@rpannek As for the history, we used to run an apt-cacher on that IP locally in the company. The vagrant-cookbook script pings this IP once and waits for 1 second for a response, if there's no response it will continue without the apt cache server. Basically, it reduced our internet usage by approximately 98% in our jenkins builds when we where very actively developing it so it made sense then.

I'm not sure where or how we build softwarecontainer these days, but maybe it should be either adapted to whatever our new location is or maybe it can be left alone with a minor comment about what happens if the server is not there, alternatively make it configurable from jenkins?