Fix inverse-proxy test cloudbuild yaml

[Bobgy]

This dockerfile assumes it is called in its containing dir as workspace. It's different from most other dockerfiles which assumes project root.

/assign @rmgogogo

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This change is 

[Bobgy]

/approve

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[Bobgy]

/test kubeflow-pipeline-sample-test

[rmgogogo]

/lgtm

thanks

[Bobgy]

Strange, this haven't fixed the issue too: https://prow.k8s.io/view/gcs/kubernetes-jenkins/pr-logs/pull/kubeflow_pipelines/2568/kubeflow-pipeline-e2e-test/1192376435893866496#1:build-log.txt%3A1298
investigating

[Bobgy]

Bug in this PR is fixed indeed, but there's a further error.
Log:

+++ dirname /opt/proxy/attempt-register-vm-on-proxy.sh
++ cd /opt/proxy
++ pwd
+ DIR=/opt/proxy
+ kubectl get configmap inverse-proxy-config
NAME                   DATA   AGE
inverse-proxy-config   3      45s
++ jq -r .data.ProxyUrl
++ kubectl get configmap inverse-proxy-config -o json
+ PROXY_URL=https://datalab-us-east1.cloud.google.com/tun/m/4592f092208ecc84946b8f8f8016274df1b36a14
++ kubectl get configmap inverse-proxy-config -o json
++ jq -r .data.BackendId
+ BACKEND_ID=
+ run-proxy-agent
+ /opt/bin/proxy-forwarding-agent --debug=false --proxy=https://datalab-us-east1.cloud.google.com/tun/m/4592f092208ecc84946b8f8f8016274df1b36a14 --proxy-timeout=60s --backend= --host=10.67.241.223:80 --shim-websockets=true --shim-path=websocket-shim --health-check-path=/ --health-check-interval-seconds=0 --health-check-unhealthy-threshold=2
2019/11/07 15:36:27 You must specify a backend ID

[Bobgy]

The new error seems to be transient, it no longer happens now. I'll keep an eye on if that's flaky.