Handle legacy image pull secrets of type kubernetes.io/dockercfg

[danielpacak]

What steps did you take and what happened:

Define image pull secret in legacy format

What did you expect to happen:

Starboard does not fail

Anything else you would like to add:

The kubernetes.io/dockercfg type is reserved to store a serialized ~/.dockercfg which is the legacy format for configuring Docker command line.

$ kubectl get sa -n starboard-operator starboard-operator -o yaml
apiVersion: v1
imagePullSecrets:
- name: starboard-operator-dockercfg-bvxj2
kind: ServiceAccount
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","kind":"ServiceAccount","metadata":{"annotations":{},"name":"starboard-operator","namespace":"starboard-operator"}}
  creationTimestamp: "2021-05-12T11:59:52Z"
  name: starboard-operator
  namespace: starboard-operator
  resourceVersion: "227356"
  selfLink: /api/v1/namespaces/starboard-operator/serviceaccounts/starboard-operator
  uid: 0634840c-bac5-43f5-90a5-07cc3d91c0fc
secrets:
- name: starboard-operator-token-brg69
- name: starboard-operator-dockercfg-bvxj2

$ kubectl get secret -n starboard-operator starboard-operator-dockercfg-bvxj2 -o yaml
apiVersion: v1
kind: Secret
metadata:
  name: starboard-operator-dockercfg-bvxj2
  namespace: starboard-operator
  annotations:
    kubernetes.io/service-account.name: starboard-operator
    kubernetes.io/service-account.uid: 0634840c-bac5-43f5-90a5-07cc3d91c0fc
    openshift.io/token-secret.name: starboard-operator-token-xp2lz
    openshift.io/token-secret.value: "*******"
  creationTimestamp: "2021-05-12T11:59:52Z"
  ownerReferences:
  - apiVersion: v1
    blockOwnerDeletion: false
    controller: true
    kind: Secret
    name: starboard-operator-token-xp2lz
    uid: fac062f9-ab94-4bd0-89f7-a8dcba4a95a4
  resourceVersion: "227353"
  selfLink: /api/v1/namespaces/starboard-operator/secrets/starboard-operator-dockercfg-bvxj2
  uid: c72a44bd-3221-47d0-8053-7ac30164f4dc
type: kubernetes.io/dockercfg
data:
  .dockercfg: e30=

Environment:

• Starboard version (use starboard version): 0.10.1
• Kubernetes version (use kubectl version): any
• OS (macOS 10.15, Windows 10, Ubuntu 19.10 etc): any