sratoolkit: add singularity files

src/singularity_modules/sratoolkit/2.8.2/build.sh

+#/bin/sh
+sudo singularity build --force bin/sratoolkit:2.8.2.sif src/singularity_modules/sratoolkit/2.8.2/sratoolkit.def
+singularity sign bin/sratoolkit:2.8.2.sif

src/singularity_modules/sratoolkit/2.8.2/sratoolkit.def

+Bootstrap: docker
+From: ubuntu:18.04
+
+%labels
+MAINTAINER Laurent Modolo
+
+%post
+SRATOOLKIT_VERSION=2.8.2
+PACKAGES=sra-toolkit=${SRATOOLKIT_VERSION}*
+
+apt-get update && \
+apt-get install -y --no-install-recommends ${PACKAGES} && \
+apt-get clean
+
+%environment
+export SRATOOLKIT_VERSION=2.8.2
+
+%runscript
+exec /bin/bash "$@"