Merge branch 'adding-new-containers' into 'master'

Adds new docker image and 0.3.5 singularity

See merge request lscsoft/bilby!344

containers/Dockerfile-bilby

+FROM bilbydev/bilby-test-suite-python37
+
+LABEL name="bilby" \
+maintainer="Gregory Ashton <gregory.ashton@ligo.org>" \
+date="20190130"
+
+RUN pip install bilby

containers/Singularity.0.3.5

+Bootstrap: docker
+From: bilbydev/bilby-test-suite-python37
+
+%help
+A singularity container for running bilby scripts. To use, simply execute the
+container, providing the bilby_script and any additional arguments. E.g.,
+
+./name_of_this_container.simg run_script.py
+
+%post
+pip install bilby==0.3.5
+
+%runscript
+exec /opt/conda/bin/python "$@"