Issue #3201358: Add live preview support

.tugboat/config.yml

+services:
+  php:
+    image: q0rban/tugboat-drupal:8.8
+    default: true
+    http: false
+    depends: mysql
+    commands:
+      update: |
+        set -eux
+        # Increase memory limit.
+        echo "memory_limit = 3G" >> /usr/local/etc/php/conf.d/my-php.ini
+        # Check out a branch using the unique Tugboat ID for this repository, to
+        # ensure we don't clobber an existing branch.
+        git checkout -b $TUGBOAT_REPO_ID
+        # This is an environment variable we added in the Dockerfile that
+        # provides the path to Drupal composer root (not the web root).
+        cd $DRUPAL_COMPOSER_ROOT
+        # We configure the Drupal project to use the checkout of the module as a
+        # Composer package repository.
+        composer config repositories.tugboat vcs $TUGBOAT_ROOT
+        # Now we can require this theme, specifing the branch name we created
+        # above that uses the $TUGBOAT_REPO_ID environment variable.
+        composer require drupal/bootstrap:dev-$TUGBOAT_REPO_ID -vvv
+
+        # Install Drupal on the site.
+        vendor/bin/drush \
+          --yes \
+          --db-url=mysql://tugboat:tugboat@mysql:3306/tugboat \
+          --site-name=Bootstrap \
+          --account-pass=admin \
+          site:install standard
+
+        # Enable the theme.
+        vendor/bin/drush --yes theme:enable bootstrap
+        # Set the theme as default.
+        vendor/bin/drush --yes config-set system.theme default bootstrap
+        # Rebuild cache.
+        vendor/bin/drush cache:rebuild
+
+        # Set up the files directory permissions.
+        mkdir -p $DRUPAL_DOCROOT/sites/default/files
+        chgrp -R www-data $DRUPAL_DOCROOT/sites/default/files
+        chmod 2775 $DRUPAL_DOCROOT/sites/default/files
+        chmod -R g+w $DRUPAL_DOCROOT/sites/default/files
+
+      build: |
+        set -eux
+        # Delete and re-check out this branch in case this is built from a Base Preview.
+        git branch -D $TUGBOAT_REPO_ID || true
+        git checkout -b $TUGBOAT_REPO_ID || true
+        cd $DRUPAL_COMPOSER_ROOT
+        composer install --optimize-autoloader
+        # Update this module, including all dependencies.
+        composer update drupal/bootstrap --with-all-dependencies
+        vendor/bin/drush --yes updb
+        vendor/bin/drush cache:rebuild
+
+  mysql:
+    image: tugboatqa/mariadb