Backport install-gosu-binary-wrapper from dockbuid

To ensure that our custom sudo wrapper is not
overwritten by a future re-install of sudo, it
is created in /usr/loca/bin

See https://github.com/dockbuild/dockbuild/issues/52

imagefiles/install-gosu-binary-wrapper.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+
+set -ex
+set -o pipefail
+
+if ! command -v gosu &> /dev/null; then
+	echo >&2 'error: "gosu" not found!'
+	exit 1
+fi
+
+# verify that the binary works
+gosu nobody true
+
+# To ensure that our custom sudo wrapper is not
+# overwritten by a future re-install of sudo, it
+# is created in /usr/loca/bin
+
+cat << EOF >> /usr/local/bin/sudo
+#!/bin/sh
+# Emulate the sudo command
+exec gosu root:root "\$@"
+EOF
+
+chmod +x /usr/local/bin/sudo
+

imagefiles/install-gosu-binary.sh

@@ -44,18 +44,3 @@ fi
 rm -r "$GNUPGHOME" /usr/local/bin/gosu.asc
 
 chmod +x /usr/local/bin/gosu
-
-# verify that the binary works
-gosu nobody true
-
-
-cat << EOF >> /usr/bin/sudo
-#!/bin/sh
-
-# Emulate the sudo command
-
-exec gosu root:root "\$@"
-
-EOF
-
-chmod +x /usr/bin/sudo