Update dashboard to 2.0.0b8 and pre-cache it again

[tstromberg]

I wasn't sure how to pre-cache their docker-hub based images, but this seems to work.

[k8s-ci-robot]

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[tstromberg]

/ok-to-test

[minikube-bot]

All Times minikube: [ 149.564599 153.186974 153.401862]
All Times Minikube (PR 6039): [ 160.350306 153.626561 158.760169]

Average minikube: 152.051145
Average Minikube (PR 6039): 157.579012

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6039) |
+----------------------+-----------+--------------------+
| minikube v           |  0.388586 |           0.382764 |
| Creating kvm2        | 55.146471 |          56.456476 |
| Preparing Kubernetes | 52.672023 |          56.745602 |
| Pulling images       | 18.507726 |          19.038475 |
| Launching Kubernetes | 22.629239 |          21.803291 |
| Waiting for cluster  |  2.648169 |           3.095343 |
+----------------------+-----------+--------------------+