Better asset compilation w/containers

Trying this:

1. Bring up container with public/assets mounted to host static assets
   directory.
2. Run 'rake assets:precompile' in the container.
3. Bring up app server container with public/assets mounted to host
   static assets directory.

This should let new assets be immediately available to an app server,
and doesn't require the app server containers to precompile assets
before bringing up the server.

WIP:  This doesn't solve anything for non-asset public files (e.g.
anything in the root public/ directory.  Those will still need to be
managed manually (until a better solution presents itself).