Switch to ninja on CI machine for faster builds
I just timed ninja vs. make on my MBP with AppleClang. A current full build takes 2m3.937s with ninja vs. 2m26.319s with make.
It's not earth-shattering, but easy to do. Just need to update the docker images to contain ninja.
Any other considerations, except changing the "cmake .." call to "cmake -GNinja .."?