feat(docker): add BACKEND_API_BASE_URL support for build-time routing

- Introduced ARG and ENV for BACKEND_API_BASE_URL in Dockerfile to ensure the correct API base URL is available during the build process.
- Updated next.config.mjs to include a warning about the implications of setting BACKEND_API_BASE_URL in the environment file, emphasizing its impact on multi-domain routing.

Dockerfile

@@ -11,6 +11,12 @@ COPY package.json pnpm-lock.yaml ./
 RUN pnpm install --frozen-lockfile
 
 FROM base AS builder
+# next.config.mjs evaluates rewrites() at build time, so the optional
+# BACKEND_API_BASE_URL override must be available here to be baked into the
+# routes manifest. Left unset, the host-based routing (opplens.com vs
+# opplenz.com) applies and no env is needed.
+ARG BACKEND_API_BASE_URL
+ENV BACKEND_API_BASE_URL=${BACKEND_API_BASE_URL}
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 RUN pnpm run build