fixes the build

pam sources are missing (they were in main)

image/prepare.sh

@@ -10,7 +10,8 @@ export INITRD=no
 mkdir -p /etc/container_environment
 echo -n no > /etc/container_environment/INITRD
 
-## Enable Ubuntu Universe and Multiverse.
+## Enable Ubuntu Universe, Multiverse, and deb-src for main.
+sed -i 's/^#\s*\(deb.*main restricted\)$/\1/g' /etc/apt/sources.list
 sed -i 's/^#\s*\(deb.*universe\)$/\1/g' /etc/apt/sources.list
 sed -i 's/^#\s*\(deb.*multiverse\)$/\1/g' /etc/apt/sources.list
 apt-get update