Store environment variables in a file, and allow init scripts to change environment variables.

Closes GH-23.

image/system_services.sh

@@ -6,6 +6,11 @@ set -x
 ## Install init process.
 cp /build/my_init /sbin/
 mkdir -p /etc/my_init.d
+mkdir -p /etc/container_environment
+touch /etc/container_environment.sh
+touch /etc/container_environment.json
+chmod 700 /etc/container_environment
+chmod 600 /etc/container_environment.sh /etc/container_environment.json
 
 ## Install runit.
 $minimal_apt_get_install runit