Including the Sphinx package in the "pip3 install" invocation causes the
docutils version requirements specified by sphinx-rtd-theme (< 0.17) to
be overridden by those specified by Sphinx (< 0.18, >= 0.14), resulting
in version 0.17.1 of docutils being installed, which is incompatible
with the latest version of sphinx-rtd-theme (0.5.2).

As sphinx-rtd-theme depends on the Sphinx package anyway, only include
the former in the "pip3 install" invocation to ensure a compatible
version of docutils is installed in the relevant Docker image.