Integrate Singularity testing into CI.

Merge request to implement the functionality described in BMK-77. Tested successfully in my own fork of the project with CI for lhcb-gen-sim and atlas-gen-bmk.

build-executor/singularity/Dockerfile

+FROM gitlab-registry.cern.ch/linuxsupport/cc7-base:latest
+
+MAINTAINER Chris Hollowell <hollowec@bnl.gov>
+
+LABEL multi.maintainer="Chris Hollowell <hollowec@bnl.gov>" \
+      multi.version="0.1" \
+      multi.description="Singularity test container for HEP workloads containers"
+
+# Append 'yum clean all' to work around https://github.com/CentOS/sig-cloud-instance-images/issues/15
+# But use '&& yum clean all' instead of '; yum clean all' to stop the build in case of errors!
+
+RUN yum clean all
+
+RUN yum install -y singularity-runtime
+RUN sed -i -e 's/underlay = no/underlay = yes/g' /etc/singularity/singularity.conf
+RUN sed -i -e 's/overlay = try/overlay = no/g' /etc/singularity/singularity.conf
+
+ARG CACHEBUST=1
+
+RUN echo CACHEBUST=$CACHEBUST
+