Fix: re-add app label to Kubernetes pods

The corresponding service otherwise won't work.

Author: LEDfan

src/main/java/eu/openanalytics/containerproxy/backend/kubernetes/KubernetesBackend.java

@@ -228,6 +228,7 @@ protected Container startContainer(ContainerSpec spec, Proxy proxy) throws Excep
 
 		Map<String, String> serviceLabels = new HashMap<>();
 		Map<String, String> podLabels = new HashMap<>();
+		podLabels.put("app", container.getId());
 
 		ObjectMetaBuilder objectMetaBuilder = new ObjectMetaBuilder()
 				.withNamespace(kubeNamespace)