On the server v0.4.21 has introduced a better way of dealing with
intra-stack networking: if the user can specify a list of endpoints
exposed internally, we now can setup a ClusterIP for this to avoid the
pitfalls of DNS-based load balancing.
This exposes the feature using the "Expose" compose field, and adds an
extra x-internal-service-type field to explicitly define how intra-stack
networking is handled on a service.
Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>
