Add job to push images from kubernetes-sigs/boskos

[ixdy]

Cloud Build configuration was added in kubernetes-sigs/boskos#8 and has been tested manually.
Staging project was created in kubernetes/k8s.io#890.

/assign @michelle192837
cc @alvaroaleman
/area boskos

[ixdy]

/retest

[ixdy]

/unassign @michelle192837
/assign @fejta
(new test-infra oncall)

[BenTheElder]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BenTheElder, ixdy

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• config/jobs/image-pushing/OWNERS [BenTheElder]

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[k8s-ci-robot]

@ixdy: Updated the job-config configmap in namespace default at cluster default using the following files:

• key k8s-staging-boskos.yaml using file config/jobs/image-pushing/k8s-staging-boskos.yaml

In response to this:

Cloud Build configuration was added in kubernetes-sigs/boskos#8 and has been tested manually.
Staging project was created in kubernetes/k8s.io#890.

/assign @michelle192837
cc @alvaroaleman
/area boskos

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