feat(helm): allow only reana-server to connect to reana-cache (#847)

Add `NetworkPolicy` that only allows reana-server to connect to
reana-cache, as other services do not rely on it.

helm/reana/templates/reana-cache.yaml

@@ -13,6 +13,23 @@ spec:
     targetPort: 6379
     protocol: TCP
 ---
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: {{ include "reana.prefix" . }}-cache
+  namespace: {{ .Release.Namespace }}
+spec:
+  podSelector:
+    matchLabels:
+      app: {{ include "reana.prefix" . }}-cache
+  policyTypes:
+    - Ingress
+  ingress:
+    - from:
+        - podSelector:
+            matchLabels:
+              app: {{ include "reana.prefix" . }}-server
+---
 apiVersion: apps/v1
 kind: Deployment
 metadata: