common.*: Optimize image size introducing "install-gosu-binary.sh" script

Script was copied from https://github.com/dockbuild/dockbuild

imagefiles/install-gosu-binary.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+set -ex
+
+if ! command -v curl &> /dev/null; then
+	echo >&2 'error: "curl" not found!'
+	exit 1
+fi
+
+if ! command -v gpg &> /dev/null; then
+	echo >&2 'error: "gpg" not found!'
+	exit 1
+fi
+
+GOSU_VERSION=1.10
+dpkgArch=$(if test $(uname -m) = "x86_64"; then echo amd64; else echo i386; fi)
+url="https://github.com/tianon/gosu/releases/download/${GOSU_VERSION}/gosu-${dpkgArch}"
+url_key="https://github.com/tianon/gosu/releases/download/${GOSU_VERSION}/gosu-${dpkgArch}.asc"
+
+# download and verify the signature
+export GNUPGHOME="$(mktemp -d)"
+
+gpg --keyserver pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4
+
+echo "Downloading $url"
+curl -o /usr/local/bin/gosu -# -SL $url
+
+echo "Downloading $url_key"
+curl -o /usr/local/bin/gosu.asc -# -SL $url_key
+
+gpg --verify /usr/local/bin/gosu.asc
+
+# cleanup
+rm -r "$GNUPGHOME" /usr/local/bin/gosu.asc
+
+chmod +x /usr/local/bin/gosu
+
+# verify that the binary works
+gosu nobody true