Docs image CI/CD script (#76)

Adds a build script that will construct and publish the `docs` image to
`quay.io`. This build script will be triggered by Google Cloud Build
on a push to `main`.

The upshot of this script is that the image used by Teleoprt CI to
run its doctests will be automatically updated every time a PR is
merged in this repository.

See-Also: gravitational/teleport#13457

.cloudbuild/image.yaml

@@ -0,0 +1,18 @@
+steps:
+  - name: 'gcr.io/cloud-builders/docker'
+    env:
+      - DOCKER_BUILDKIT=1
+      - DOCKER_REGISTRY=quay.io
+      - DOCKER_REPOSITORY=gravitational/docs
+      - DOCKER_TAG=latest
+    secretEnv:
+      - QUAYIO_DOCKER_USERNAME
+      - QUAYIO_DOCKER_PASSWORD
+    script: .cloudbuild/publish-image.sh 
+availableSecrets:
+  secretManager:
+  - versionName: projects/771512790633/secrets/ci_docs-quay_io_username/versions/1
+    env: QUAYIO_DOCKER_USERNAME
+  - versionName: projects/771512790633/secrets/ci_docs_quay_io_password/versions/1
+    env: QUAYIO_DOCKER_PASSWORD
+

.cloudbuild/publish-image.sh

@@ -0,0 +1,7 @@
+#! /bin/env bash
+set -e
+IMAGE_NAME=$DOCKER_REGISTRY/$DOCKER_REPOSITORY:$DOCKER_TAG
+echo Building $IMAGE_NAME
+DOCKER_BUILDKIT=1 docker build -t $IMAGE_NAME .
+echo $QUAYIO_DOCKER_PASSWORD | docker login -u $QUAYIO_DOCKER_USERNAME --password-stdin $DOCKER_REGISTRY
+docker push $IMAGE_NAME