Create CD script to be run after every push to main branch

Hook up guestbook-go (first) to push images to the staging project
with prow.

dev/cd/after-push-to-branch

@@ -0,0 +1,37 @@
+#!/bin/bash
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+# cd to the repo root
+REPO_ROOT=$(git rev-parse --show-toplevel)
+cd "${REPO_ROOT}"
+
+if [ -z "${GIT_REF:-}" ]; then
+  echo "GIT_REF must be set"
+  exit 1
+fi
+
+if [[ -z "${REGISTRY_BASE:-}" ]]; then
+  echo "REGISTRY_BASE must be set"
+  exit 1
+fi
+
+if [[ ! "${GIT_REF}" =~ ^refs/heads/.* ]]; then
+  echo "GIT_REF=${GIT_REF} is not of the expected format refs/heads/*"
+  exit 1
+fi
+
+BRANCH=${GIT_REF/refs\/heads\//}
+echo "BRANCH is ${BRANCH}"
+
+GIT_REVISION=$(git rev-parse --short HEAD)
+echo "GIT_REVISION is ${GIT_REVISION}"
+
+export IMAGE_TAG="g${GIT_REVISION}"
+echo "IMAGE_TAG is ${IMAGE_TAG}"
+
+pushd ${REPO_ROOT}/guestbook-go
+VERSION=${IMAGE_TAG} REGISTRY=${REGISTRY_BASE} make push
+popd

dev/cloudbuild/cloudbuild.yaml

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+timeout: 900s
+steps:
+  - name: 'gcr.io/k8s-staging-test-infra/gcb-docker-gcloud:v20230111-cd1b3caf9c'
+    entrypoint: dev/cd/after-push-to-branch
+    env:
+    - GIT_REF=$_GIT_TAG
+    - PULL_BASE_REF=$_PULL_BASE_REF
+    - REGISTRY_BASE=gcr.io/k8s-staging-examples
+substitutions:
+  # _GIT_TAG will be filled with a git-based tag for the image, of the form vYYYYMMDD-hash, and
+  # can be used as a substitution
+  _GIT_TAG: '12345'
+  _PULL_BASE_REF: 'master'