containerized ceph builds

Dear Community, In several occasions I ran into the need of trying to build my code in different OS environments. So, instead of spinning VMs, I decided to try containerized builds. Some tweaks were needed in the Docker files, but overall I managed to build the latest master against centos7, ubuntu18, and fedora31. Feel free to try it out from here [1]. Note that your local code is mounted - no cloning inside the container. Just the "build" directory is inside the container (so it can be done in parallel to your regular "build" dir). Also, note that centos 8 does not work yet :-( Yuval [1] https://github.com/yuvalif/ceph-builder