Create a Singularity file with gdown.pl (#15)

Singularity

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+BootStrap: docker
+FROM: perl
+
+# This is the script that's executed when you don't call it with any script or shell to execute.
+%runscript
+        exec /usr/local/bin/gdown.pl "$@"
+
+%setup
+	cp gdown.pl ${SINGULARITY_ROOTFS}/usr/local/bin/
+
+# These commands will be executed inside of the container during building.
+%post
+	apt-get update
+	apt install -y wget