Improve CORS support in UI backend

[jackie-ob]

In deployments where UI static and UI backend services have different origins, preflight CORS checks fail. This is true if the deployment architecture does not include any HTTP level routing layer that pulls UI static + UI backend services under a single origin (e.g. with k8s ingress, or other ALB). E.g. for local development, running on localhost.

See: https://share.getcloudapp.com/BluGrpKB

We add an env var ORIGIN_TO_ALLOW_CORS_FROM to UI static backend so allow such deployments to work correctly. E.g. if UI backend shall be hosted on http://localhost:3000 (with UI backend hosted elsewhere, like http://localhost:8083), then set this in UI backend service's environment:

ORIGIN_TO_ALLOW_CORS_FROM="http://localhost:3000".

Note that it may be set to * as well, if permitting all origins is OK.