fix: add explicit ARG/ENV VITE_API_URL to Dockerfile

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.

apps/web/Dockerfile

@@ -11,6 +11,8 @@ RUN pnpm install --frozen-lockfile
 
 # Build
 FROM deps AS builder
+ARG VITE_API_URL=
+ENV VITE_API_URL=
 COPY packages/types/ packages/types/
 COPY apps/web/ apps/web/
 RUN pnpm --filter @groombook/web build