Many of you probably know that we currently have `support/docker-build.sh` to power our CI for our various configurations. One of the problems for us has been that we create a `Dockerfile` ad-hoc and invoke `docker build` with it. This is very inefficient and also leads to flaky issues around `apt-get install`.
I've introduced `support/mesos-build.sh` which operates off of docker images hosted on Dockerhub instead, and should aid in bringing us faster and more stable CI results! As a bonus, we now also test Clang on the CentOS 7! Thanks, MPark
