Overview
Example app.yaml:
runtime: go113
env_variables:
DATASTORE_PROJECT_ID: "PROJECT_ID"
DATASTORE_PROJECT_ID should be something like "opencircuits-beta" - whatever the project id for you GCP project is.
To change projects with gcloud use gcloud init
Add domain name to project: gcloud --project=<your_project_id> beta app domain-mappings create <domain>