[PATCH v3 4/6] DO-NOT-MERGE: testing: add pip-installed sphinx-build to CentOS 8

[John Snow]

This is just for sake of demonstration again; by modifying the
dockerfile we can continue to support building docs on CentOS 8 with
minimal pain.

Note: This uses pip to install packages as root; this is hygienically
not a *great* idea for arbitrary systems. For CI, I am abusing the fact
that this is a custom-built environment that does not get altered after
being built. Individual developers should almost certainly just use a
virtual environment to avoid interfering with their system.

That process can look like this:

1. Install the new python interpreter:
   > sudo dnf install python38

2. Go to a folder where you'd be happy to store some python crud:
   > cd ~/.cache

3. Create a virtual environment using the new python:
   > python3.8 -m venv qemu_venv

4. Install a modern version of sphinx into this venv:
   > ./qemu_venv/bin/pip install sphinx

5. Try running sphinx-build:
   > ./qemu_venv/bin/sphinx-build --help

>From here, you can specify ~/.cache/qemu_venv/bin/sphinx-build as your
sphinx binary to configure in order to get doc building. This approach
doesn't interfere with anything; it requires you specifically to opt
into using it, which is likely the safest option if you don't have a lot
of Python knowhow.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 tests/docker/dockerfiles/centos8.docker | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tests/docker/dockerfiles/centos8.docker 
b/tests/docker/dockerfiles/centos8.docker
index a3bfddf382d..a53ccada55a 100644
--- a/tests/docker/dockerfiles/centos8.docker
+++ b/tests/docker/dockerfiles/centos8.docker
@@ -129,6 +129,10 @@ RUN dnf distro-sync -y && \
     ln -s /usr/bin/ccache /usr/libexec/ccache-wrappers/g++ && \
     ln -s /usr/bin/ccache /usr/libexec/ccache-wrappers/gcc
 
+RUN /usr/bin/python3.8 -m ensurepip && \
+    /usr/bin/python3.8 -m pip --no-cache-dir install sphinx sphinx-rtd-theme
+
+
 ENV CCACHE_WRAPPERSDIR "/usr/libexec/ccache-wrappers"
 ENV LANG "en_US.UTF-8"
 ENV MAKE "/usr/bin/make"
-- 
2.39.0