dowload jenkins.war, no need for debian package

the later include daemon setup that doesn’t make sense in a Docker container
also require to register custom apt-key, just to get the jenkins package available.
but we only need a war !
Does Jenkins actually use this file, or recommend that it be in place?  It's not being put somewhere deterministic, just the "current" directory (which is likely to be just `/`), so I'm not really seeing what this is providing and I think I must've missed something important.