Environment variables:
GCS_BUCKET - GCS bucket to point at.
ROUTING - routing to use. Possible values: react, gatsby, disabled.
PATH_PREFIX - Bucket URL path prefix, defaults to empty string.
INDEX - index path. Default: index.html.
PORT - port to listen at. Default: 80.
FORCE_HTTPS - If defined, will redirect http to https with 301 http code.
ERROR_404 - error 404 handling. Not available when routing is react. Default: /404.html.