handle --net=container:<image> dependencies

As I feared, it turns out there the detection image/id is done at runtime, by doing 2 separate lookups: https://github.com/dotcloud/docker/blob/a091d94/daemon/daemon.go#L115-L126. We can't really do that since this call may happen before the dependency is started.

So either:
1. we use some heuristics to detect an id (hexadecimal chars & a certain length);
2. we check whether the reference is a name by looking it up in the config, but IMHO it breaks the separation of concerns (and might result in a false negative just because a target is cutting out that dependency, something which should be detected in container_map.go, and not blow up so early in the process);
3. we don't do anything about it except mentioning that it's not supported in the documentation.

I tend to thing 3. is the best approach since neither 1. or 2. is bullet-proof.