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/pause.go

@@ -0,0 +1,8 @@
+package main
+
+import "syscall"
+
+func main() {
+	// Halts execution, waiting on signal.
+	syscall.Pause()
+}