Add ability to output json lists in metadata build file

Signed-off-by: Laurent Goderre <laurent.goderre@docker.com>

commands/build.go

@@ -763,8 +763,11 @@ func decodeExporterResponse(exporterResponse map[string]string) map[string]inter
 		}
 		var raw map[string]interface{}
 		if err = json.Unmarshal(dt, &raw); err != nil || len(raw) == 0 {
-			out[k] = v
-			continue
+			var rawList []map[string]interface{}
+			if err = json.Unmarshal(dt, &rawList); err != nil || len(rawList) == 0 {
+				out[k] = v
+				continue
+			}
 		}
 		out[k] = json.RawMessage(dt)
 	}