security archives for images.
If the image being built is jessie or stretch, add security apt sources. I considered moving to using 'stable', 'testing' all over the place instead of the code-names, but this would have the undesired effect of changing releases automatically when the code names roll over. I looked for a way of mapping testing->stretch etc in a more future proof fashion but came up short. /etc/debian_version is not useful for this. Closes: #9, #10.