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.
Why :latest?  Also, it would be good to pull this out into a constant that can be specified at build time via a ld flag (like git version is).