builder: add an option for specifying build target

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

image_build.go

@@ -95,6 +95,7 @@ func (cli *Client) imageBuildOptionsToQuery(options types.ImageBuildOptions) (ur
 	query.Set("cgroupparent", options.CgroupParent)
 	query.Set("shmsize", strconv.FormatInt(options.ShmSize, 10))
 	query.Set("dockerfile", options.Dockerfile)
+	query.Set("target", options.Target)
 
 	ulimitsJSON, err := json.Marshal(options.Ulimits)
 	if err != nil {