azure: docurium: fix build failure due to bumped CMake requirements

Our Docurium builds currently depend on Debian Jessie, which has CMake
v3.0 available. As rugged has bumped its CMake requirements to need at
least v3.5 now, the documentation build is thus failing.

Fix this by converting our Docurium Docker image to be based on Ubuntu
Bionic. We already do base all of our images on Ubuntu, so I don't see
any sense in using Debian here. If this was only to speed up builds, we
should just go all the way and use some minimal container like Alpine
anyway.

Also remove cache busters. As we're rebuilding the image every time,
it's we really don't need them at all.

azure-pipelines/docker/docurium

-FROM debian:jessie-slim
-ARG CACHEBUST=1
-RUN apt-get update
-RUN apt install -y cmake pkg-config ruby ruby-dev llvm libclang-3.5-dev libssl-dev python-pygments
-ARG CACHEBUST=1
+FROM ubuntu:bionic
+RUN apt update && apt install -y cmake pkg-config ruby ruby-dev llvm libclang-dev libssl-dev python-pygments
 RUN gem install docurium