Without this, Vite sees VITE_API_URL as undefined (not empty string) at
build time. The ?? operator only replaces null/undefined, not a missing var,
so better-auth receives undefined — which it treats as a relative path and
prepends window.location.origin at build time, resulting in the UAT URL being
baked in.
Explicitly setting ARG VITE_API_URL= (empty string) in the Dockerfile makes
Vite see it as defined with empty value, so the || fallback fires at runtime.
Fixes GRO-1280.
Chris Farhood
c2dd1dbf84