(If you've never had to use Helm, I envy you. And if you have, I genuinely look forward to you showing me an easier way to do this, since it would make my life easier.)

-------------------------------------

Shell script:

  iptables -A INPUT -p tcp --dport 8080 -j ACCEPT

  for port in 80 443 8080; do
    iptables -A INPUT -p tcp --dport "$port" -j ACCEPT
  done

-------------------------------------

Kubernetes (disclaimer: untested, obviously)

  apiVersion: networking.k8s.io/v1
  kind: NetworkPolicy
  spec:
    trafficPolicy:
      firewall:
        rules:
        - name: allow-port-8080
          ports:
          - port: 8080
            protocol: TCP
    podSelector:
      matchLabels:
        app.kubernetes.io/name: my-app

  firewall:
    rules:
      - name: allow-port-80
        ports:
          - port: 80
            protocol: TCP
      - name: allow-port-443
        ports:
          - port: 443
            protocol: TCP
      - name: allow-port-8080
        ports:
          - port: 8080
            protocol: TCP

  apiVersion: networking.k8s.io/v1
  kind: NetworkPolicy
  metadata:
    name: firewall
  spec:
    trafficPolicy:
      firewall:
        rules:
          {{- range .Values.firewall.rules }}
          - name: {{ .name }}
            ports:
            {{- range .ports }}
            - port: {{ .port }}
              protocol: {{ .protocol }}
            {{- end }}
          {{- end }}
    podSelector:
      matchLabels:
        app.kubernetes.io/name: my-app