Add alpha test of a CentOS 8 container

docker-c8/Dockerfile

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+FROM centos:8
+
+RUN dnf -y install zip curl
+
+# Create all possible pool accounts
+RUN curl -s https://raw.githubusercontent.com/stfc/grid-workernode/master/resources/pool_accounts/create.sh | bash -
+
+# CVMFS (for SUM tests only; install before adding other repos to ensure we have the latest version)
+RUN dnf -y install https://ecsft.cern.ch/dist/cvmfs/cvmfs-release/cvmfs-release-latest.noarch.rpm \
+                   https://download.opensuse.org/repositories/home:/cvmfs:/contrib-egi/CentOS_7/noarch/cvmfs-config-egi-2.4-1.10.obs.el7.noarch.rpm
+
+# EPEL
+RUN dnf -y install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
+
+# Basic dependencies
+RUN dnf -y install freetype expat gcc glibc-headers time man unzip quota attr tcsh
+
+# Boost (need to ensure gfal CLI from CVMFS will work for some VOs)
+RUN dnf -y install boost-date-time boost-filesystem boost-graph boost-iostreams boost-program-options \
+                   boost-regex boost-serialization boost-signals boost-system boost-test \
+                   boost-thread boost-wave
+
+# Singularity
+RUN dnf -y install singularity
+
+# Disable overlay and loop device management in singularity
+RUN sed -i '/enable overlay/c\enable overlay = no' /etc/singularity/singularity.conf && \
+    sed -i '/enable underlay/c\enable underlay = yes' /etc/singularity/singularity.conf && \
+    sed -i '/max loop devices/c\max loop devices = 0' /etc/singularity/singularity.conf
+
+# Update & cleanup
+RUN dnf -y update && \
+    dnf clean all && \
+    rm -rf /var/cache/dnf