fix: openvpn/start.sh add condition to prevent appending duplicative name server entries

[garretpremo]

This PR adds a simple check to prevent writing duplicate entries to resolv.conf

The issue:
When deploying this image in Kubernetes, using openVPN, in some situations the container would restart without recreating.

This would cause the start.sh script to run a second time, and the resolv.conf would be written to for a second time. Having duplicate entries in the resolv.conf left the container in a permanent, unrecoverable error state until the pod was manually recreated.

Node Specs:

Capacity:
  cpu:                16
  memory:             32765604Ki
System Info:
  Kernel Version:             6.8.0-50-generic
  OS Image:                   Ubuntu 24.04.1 LTS
  Operating System:           linux
  Architecture:               amd64
  Container Runtime Version:  containerd://1.7.22
  Kubelet Version:            v1.31.1
  Kube-Proxy Version:         v1.31.1

Below is the deployment configuration for anyone looking to recreate this:

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: qbittorrent
  name: qbittorrent
spec:
  replicas: 1
  selector:
    matchLabels:
      app: qbittorrent
  strategy: {}
  template:
    metadata:
      labels:
        app: qbittorrent
    spec:
      securityContext:
        fsGroup: 65534
        runAsUser: 0
      containers:
      - image: dyonr/qbittorrentvpn
        imagePullPolicy: IfNotPresent
        name: qbittorrentvpn
        env:
        - name: VPN_ENABLED
          value: "true"
        - name: VPN_TYPE
          value: openvpn
        - name: LAN_NETWORK
          value: "192.168.0.0/16"
        - name: NAME_SERVERS
          value: "1.1.1.1,1.0.0.1"
        envFrom:
        - secretRef:
            name: qbittorrent-secrets # contains "VPN_USERNAME", "VPN_PASSWORD"
        securityContext:
          privileged: true  # probably not needed
          capabilities:
            add: ["NET_ADMIN", "NET_RAW"] # probably do not need `NET_RAW`
        resources: {}
        ports:
        - name: webui
          containerPort: 8080
        - name: tcplistening
          containerPort: 8999
        - name: udplistening
          containerPort: 8999
          protocol: UDP
        volumeMounts:
        - mountPath: /config
          name: config
        - mountPath: /downloads
          name: downloads

      volumes:
      - name: config
        persistentVolumeClaim:
          claimName: qbittorrent-config
      - name: downloads
        persistentVolumeClaim:
          claimName: qbittorrent-downloads
status: {}