Add nightly Pipelines release 🌙

This is the configuration to run the Pipeline + Tasks being added in
tektoncd/pipeline#1274 every night at midnight
PDT.

I've successfully run this by setting the cron to every 5 minutes. This
configuration is already applied to the repo.

Status can be seen at https://prow.tekton.dev/?type=periodic.

Part of tektoncd/pipeline#860

prow/README.md

@@ -193,11 +193,17 @@ kubectl apply --filename  https://storage.googleapis.com/tekton-releases/previou
 
 _See also [Tekton Pipelines installation instructions](https://github.com/tektoncd/pipeline/blob/master/docs/install.md)._
 
+#### Nightly Tekton Pipelines release
+
+The prow configuration includes a `periodic` job which invokes
+[the Tekton Pipelines nightly release Pipeline](https://github.com/tektoncd/pipeline/tree/master/tekton#nightly-releases).
+
 #### Hello World Pipeline
 
 Since Prow + Pipelines in this org are a WIP (see
 [#922](https://github.com/tektoncd/pipeline/issues/922)),
-the only job that is currently configured is
+the only job (besides [nightly releases](#nightly-tekton-pipelines-release))
+that is currently configured is
 [the hello scott Pipeline](prow/helloscott.yaml).
 
 This `Pipeline` (`special-hi-scott-pipeline`) is executed on every PR to this repo
@@ -219,4 +225,4 @@ kubectl create configmap config --from-file=config.yaml=prow/config.yaml --dry-r
 
 # Step 3: Remember to configure kubectl to connect to your regular cluster!
 gcloud container clusters get-credentials ...
-```
+```

prow/config.yaml

@@ -1107,3 +1107,66 @@ periodics:
         - name: test-account
           secret:
             secretName: test-account
+# Nightly release for Tekton Pipelines: https://github.com/tektoncd/pipeline/tree/master/tekton#nightly-releases
+- cron: "0 0  * * *"
+  name: tekton-pipelines-nightly-release
+  agent: tekton-pipeline
+  extra_refs:
+  - org: tektoncd
+    repo: pipeline
+    base_ref: master
+  pipeline_run_spec:
+    trigger:
+      type: manual
+    serviceAccount: "release-right-meow"
+    pipelineRef:
+      name: pipeline-release-nightly
+    resources:
+    - name: bucket
+      resourceRef:
+        name: tekton-bucket-nightly
+    - name: source-repo
+      resourceRef:
+        name: PROW_EXTRA_GIT_REF_0
+    - name: builtKoImage
+      resourceRef:
+        name: ko-image
+    - name: builtBaseImage
+      resourceRef:
+        name: base-image
+    - name: builtEntrypointImage
+      resourceRef:
+        name: entrypoint-image
+    - name: builtKubeconfigWriterImage
+      resourceRef:
+        name: kubeconfigwriter-image
+    - name: builtCredsInitImage
+      resourceRef:
+        name: creds-init-image
+    - name: builtGitInitImage
+      resourceRef:
+        name: git-init-image
+    - name: builtNopImage
+      resourceRef:
+        name: nop-image
+    - name: builtBashImage
+      resourceRef:
+        name: bash-image
+    - name: builtGsutilImage
+      resourceRef:
+        name: gsutil-image
+    - name: builtControllerImage
+      resourceRef:
+        name: controller-image
+    - name: builtWebhookImage
+      resourceRef:
+        name: webhook-image
+    - name: builtDigestExporterImage
+      resourceRef:
+        name: digest-exporter-image
+    - name: builtPullRequestInitImage
+      resourceRef:
+        name: pull-request-init-image
+    - name: builtGcsFetcherImage
+      resourceRef:
+        name: gcs-fetcher-image