packer/alpine-linux: missing shabangs

Packer does not care but they do not hurt anyone

packer/alpine-linux/scripts/frr.sh

@@ -1,3 +1,5 @@
+#!/bin/sh
+
 # add community repository
 sed -i 's/^#\s*\(.*\/v.*\/community\)$/\1/' /etc/apk/repositories
 apk update

packer/alpine-linux/scripts/post_setup.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 # restore default sshd configuration
 mv /etc/ssh/sshd_config.orig /etc/ssh/sshd_config
 

packer/alpine-linux/scripts/setup.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 # install additional packages
 apk add nano busybox-extras