Tweak which Docker images are used

For CL, use images from clfoundation/ instead of mitmers/. The source code for
these images is found at

For other tasks, use buildpack-deps. This image has git and make already
installed, additionally the CL images are based on buildpack-deps. This should
speed up jobs somewhat since fewer packages need to be installed.
6 jobs for ci-tweaks in 21 minutes and 40 seconds (queued for 3 seconds)