Add zeus-mcmc to docker files

This includes zeus in the docker files (refs !962 (merged)).

Note that zeus is for Python 3.6+, so I've edited the write_dockerfiles.py to exclude it from the Python 3.5 image. We may instead just want to remove the Python 3.5 file altogether as it's not used in the CI testing?

containers/dockerfile-template

@@ -34,7 +34,7 @@ RUN pip install sphinx numpydoc nbsphinx sphinx_rtd_theme sphinx-tabs autodoc
 
 # Install dependencies and samplers
 RUN pip install corner lalsuite theano healpy cython tables
-RUN pip install cpnest dynesty emcee nestle ptemcee pymc3 kombine dnest4 nessai
+RUN pip install cpnest dynesty emcee nestle ptemcee pymc3 kombine dnest4 nessai zeus-mcmc
 RUN conda install -n ${{conda_env}} -c conda-forge pymultinest ultranest
 
 # Install Polychord