Add website staging GCB template (#5418)

* Adds a GCB template that is responsible for building the website and
staging it to firebase in PRs
* Once the website is built, `dart-github-bot` will comment in the PR a
link that you can navigate to in order to see your changes.
* After this template is merged in, we can update the PR trigger to run
this template on all PRs. It is currently disabled in order to ensure
that it is not run on branches without this template in it yet.

cloud_build/scripts/stage_site_and_comment_on_github.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+set -e
+
+deploy_to_firebase_staging_channel () {
+    echo "Deploying website to a staging channel on firebase..."
+
+    # Deploy to firebase, but save the output to a variable so we can grep the
+    # url from the output and save it to FIREBASE_STAGING_URL for future use.
+    FIREBASE_DEPLOY_RESPONSE=$(firebase hosting:channel:deploy --expires 7d pr$PR_NUMBER-$HEAD_BRANCH --project=$PROJECT_ID)
+    echo "$FIREBASE_DEPLOY_RESPONSE"
+    FIREBASE_STAGING_URL=$(grep -Eo "https://$PROJECT_ID--[a-zA-Z0-9./?=_%:-]*" <<< "$FIREBASE_DEPLOY_RESPONSE")
+}
+
+login_to_github() {
+    echo "Logging into github under bot account..."
+
+    echo $GH_PAT_TOKEN > token
+    gh auth login --with-token < token
+}
+
+comment_staging_url_on_github () {
+    echo "Commenting staging url on the PR..."
+    COMMENT_BODY=$(echo -e "Visit the preview URL for this PR (updated for commit $COMMIT_SHA):\n\n$FIREBASE_STAGING_URL")
+
+    # The github CLI throws an error if --edit-last doesn't find a previous
+    # comment, so this edits the last comment, but if it doesn't exist,
+    # leave a new comment.
+    gh pr comment $PR_NUMBER --edit-last --body "$COMMENT_BODY" --repo $REPO_FULL_NAME || \
+        gh pr comment $PR_NUMBER --body "$COMMENT_BODY" --repo $REPO_FULL_NAME
+}
+
+deploy_to_firebase_staging_channel
+login_to_github
+comment_staging_url_on_github

cloud_build/stage.yaml

@@ -0,0 +1,32 @@
+steps:
+  - name: gcr.io/cloud-builders/git
+    args: ['submodule', 'update', '--init', '--recursive']
+  - name: gcr.io/cloud-builders/docker
+    entrypoint: '/bin/bash'
+    args:
+      - '-c'
+      - |-
+        set -e
+
+        echo "Building the website using a makefile..."
+        make build
+  - name: gcr.io/flutter-dev-230821/firebase-staging
+    entrypoint: '/bin/bash'
+    args:
+      - '-c'
+      - |-
+        cloud_build/scripts/stage_site_and_comment_on_github.sh
+    secretEnv: ['GH_PAT_TOKEN']
+    env:
+      - 'PR_NUMBER=$_PR_NUMBER'
+      - 'HEAD_BRANCH=$_HEAD_BRANCH'
+      - 'PROJECT_ID=$PROJECT_ID'
+      - 'COMMIT_SHA=$COMMIT_SHA'
+      - 'REPO_FULL_NAME=$REPO_FULL_NAME'
+availableSecrets:
+  secretManager:
+  - versionName: projects/$PROJECT_ID/secrets/dart-github-bot-comment-pat/versions/latest
+    env: 'GH_PAT_TOKEN'
+timeout: 1200s
+options:
+  logging: CLOUD_LOGGING_ONLY