skip test_not_writable_ipdir if I can't create a non-writable dir

such as when running as root

just came across this running iptest in a docker container. This was the only test that failed.
run iptest in Dockerfile

and install sphinx with apt, since it's super slow
because of 2to3
Merge pull request #6546 from minrk/root-no-write

run iptest in Dockerfile