Comment 4 for bug 1699913

@mwhudson I'm concerned about https://bazaar.launchpad.net/~mwhudson/livecd-rootfs/lp-1699913/revision/1675 -- this leaves around the index files that aren't going to change, but I think this is going to cause subtle security issues for some users, isn't it?

With this change, "RUN apt-get install -y git" will install old vulnerable git while "RUN apt-get update && apt-get install -y git" will install security-updated git, which is the other half of why we remove the lists entirely in the base image (IMO, getting "no such package" is a much better indicator of the need for "apt-get update" than just getting an outdated/insecure package quietly).