Tweak which Docker images are used
For CL, use images from clfoundation/ instead of mitmers/. The source code for these images is found at https://gitlab.common-lisp.net/cl-docker-images/ 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.