Update Cloudbuild to set BaseURL
Update Cloudbuild to set BaseURL

Adds a step in build that uses `sed` to replace the base value
with a Cloud Build Substitution value

GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app