Use buffered chan to avoid losing docker events

Docker imposes a limit that events have to be retrieved within 100ms
or they are dropped on the sending side. Adding buffer space within
the Weave Net process allows those events to be collected from Docker
more quickly.

1024 is an arbitrary number, much higher than we expect to need.

common/docker/client.go

@@ -99,7 +99,9 @@ func (c *Client) AddObserver(ob ContainerObserver) error {
 		pending := make(pendingStarts)
 		retryInterval := InitialInterval
 		for {
-			events := make(chan *docker.APIEvents)
+			// Use a buffered chan so the client library can run ahead of the listener
+			// - Docker will drop an event if it is not collected quickly enough.
+			events := make(chan *docker.APIEvents, 1024)
 			if err := c.AddEventListener(events); err != nil {
 				c.errorf("Unable to add listener to Docker API: %s - retrying in %ds", err, retryInterval/time.Second)
 			} else {