Add a pause image for the net container.

The pause image is a 240KB image that simply pauses waiting on a signal.
Use this for the net container which only needs to act as a placeholder.
Current net image is ~2.5MB. From my tests, this reduces startup time
for the net container from ~14s to ~6s.

build/pause/prepare.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -e
+set -x
+
+go build --ldflags '-extldflags "-static" -s' pause.go