Don't create a HostConfig accidentally if none was passed in

proxy/start_container_interceptor.go

@@ -12,22 +12,25 @@ func (i *startContainerInterceptor) InterceptRequest(r *http.Request) error {
 		return err
 	}
 
+	// If the client has sent some JSON which might be a HostConfig, add our
+	// parameters back into it, otherwise Docker will consider them overwritten
 	if containerShouldAttach(container) && r.Header.Get("Content-Type") == "application/json" {
 		hostConfig := map[string]interface{}{}
 		if err := unmarshalRequestBody(r, &hostConfig); err != nil {
 			return err
 		}
+		if len(hostConfig) > 0 {
+			i.proxy.addWeaveWaitVolume(hostConfig)
+			if dnsDomain := i.proxy.getDNSDomain(); dnsDomain != "" {
+				if err := i.proxy.setWeaveDNS(hostConfig, container.Config.Hostname, dnsDomain); err != nil {
+					return err
+				}
+			}
 
-		i.proxy.addWeaveWaitVolume(hostConfig)
-		if dnsDomain := i.proxy.getDNSDomain(); dnsDomain != "" {
-			if err := i.proxy.setWeaveDNS(hostConfig, container.Config.Hostname, dnsDomain); err != nil {
+			if err := marshalRequestBody(r, hostConfig); err != nil {
 				return err
 			}
 		}
-
-		if err := marshalRequestBody(r, hostConfig); err != nil {
-			return err
-		}
 	}
 	i.proxy.createWait(r, container.ID)
 	return nil