Improve handling of build container
Improve handling of build container

- Only mention the need for it on the one rule where it gets used.
- Don't delete it in `make clean-bin`.

weaveworks/weave,n054/weave,paulbellamy/weave,weaveworks/weave,weaveworks/weave,paulbellamy/weave,paulbellamy/weave,paulbellamy/weave,n054/weave,n054/weave,weaveworks/weave,n054/weave,n054/weave