...but don't push

I'm hoping this will fail the docker build, vs
systemed#625, where it passed.

Once it does, then I'll try to actually fix the Docker build.

This is very much a monkey-see/monkey-do attempt to fix the Docker build
based on a quick Google. I'm guessing libprotobuf previously dragged in zlib.

(This also suggests that the other install docs might now be inaccurate -
I'll try running on a clean Ubuntu to check.)