gatk: add singularity files

src/singularity_modules/gatk/4.0.8.1/build.sh

+#/bin/sh
+sudo singularity build --force bin/gatk:4.0.8.1.sif src/singularity_modules/gatk/4.0.8.1/gatk.def
+singularity sign bin/gatk:4.0.8.1.sif

src/singularity_modules/gatk/4.0.8.1/gatk.def

+Bootstrap: docker
+From: broadinstitute/gatk:4.0.8.1
+
+%labels
+MAINTAINER Laurent Modolo
+
+%post
+
+GATK_VERSION=4.0.8.1
+cp gatk/gatk /usr/bin/
+
+%environment
+export GATK_VERSION=4.0.8.1
+
+%runscript
+exec /bin/bash "$@"