Clean up `apt` configuration and cache
This MR removes the Debian stretch package configuration after it's been used to install git, and also removes cached .deb files from the generated image.
This will hopefully make the build images both smaller, and easier to experimentally add packages to in MRs. Currently, a line like apt-get install foo in a WIP MR for a newly required package will cause the stretch version to be installed. I believe this is leading to segfaults in https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2029
Obviously, we don't want people installing random packages as part of the CI run - that's why we have these images - but it's useful for validating an approach before opening an MR to add the package to the image.