Note: we no longer need to detect awsvpc when launching.
The "AWS VPC" mode is now set via the `--awsvpc` flag.
Before we had to "detect" it when the proxy was potentially started
later, after the router had started.
We drop WEAVEPROXY_DOCKER_ARGS as obsolete.
They seem to stop Docker using its build cache.
Removing the line entirely stops weaveproxy from working, as it
assumes it can exec `./weave` from the current directory.
Bryan Boreham