Add Container image build automation

As suggested by BenTheElder, we need to add a cloudbuild.yaml spec that
will be used by the prow job to automate the container build and push
the tags.

Signed-off-by: Arnaud Meukam <ameukam@gmail.com>

Makefile

@@ -6,8 +6,9 @@ NETLIFY_FUNC      = $(NODE_BIN)/netlify-lambda
 # but this can be overridden when calling make, e.g.
 # CONTAINER_ENGINE=podman make container-image
 CONTAINER_ENGINE ?= docker
+IMAGE_REGISTRY ?= gcr.io/k8s-staging-sig-docs
 IMAGE_VERSION=$(shell scripts/hash-files.sh Dockerfile Makefile | cut -c 1-12)
-CONTAINER_IMAGE   = kubernetes-hugo:v$(HUGO_VERSION)-$(IMAGE_VERSION)
+CONTAINER_IMAGE   = $(IMAGE_REGISTRY)/k8s-website-hugo:v$(HUGO_VERSION)-$(IMAGE_VERSION)
 CONTAINER_RUN     = $(CONTAINER_ENGINE) run --rm --interactive --tty --volume $(CURDIR):/src
 
 CCRED=\033[0;31m

cloudbuild.yaml

@@ -0,0 +1,24 @@
+# See https://cloud.google.com/cloud-build/docs/build-config
+
+# this must be specified in seconds. If omitted, defaults to 600s (10 mins)
+timeout: 1200s
+# this prevents errors if you don't use both _GIT_TAG and _PULL_BASE_REF,
+# or any new substitutions added in the future.
+options:
+  substitution_option: ALLOW_LOOSE
+steps:
+  - name: "gcr.io/k8s-testimages/gcb-docker-gcloud:v20190906-745fed4"
+    entrypoint: make
+    env:
+      - DOCKER_CLI_EXPERIMENTAL=enabled
+      - TAG=$_GIT_TAG
+      - BASE_REF=$_PULL_BASE_REF
+    args:
+      - container-image
+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 will contain the ref that was pushed to to trigger this build -
+  # a branch like 'master' or 'release-0.2', or a tag like 'v0.2'.
+  _PULL_BASE_REF: "master"