Usability fixes to CI runner script

[driazati]

This fixes the CPU-only docs build (which now runs successfully on the ci-cpu image, tested on a fresh vm via Vagrant) and adds some hints for fixing common errors (and saves them until errors actually occur so they aren't mixed into the long log somewhere).

• check that a gpu is present when --cpu is not used
• check that docker is installed
• check if the build directory exists (e.g. I use ninja outside of Docker but docker uses the same folder to build and wants to generate Makefiles, which makes cmake error out)
• shorten docker commands by running a file that is also set -x'ed
• fix help printing

cc @areusch

[areusch]

thanks for these usability improvements @driazati!