fix: set API_BASE_URL at Docker build time for Next.js inlining

Next.js replaces process.env.X with build-time values via
DefinePlugin, even in server-side route handlers. Without
the env var set during build, it always falls back to
localhost:8000 which doesn't work inside Docker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

it0-web-admin/Dockerfile

@@ -8,6 +8,11 @@ RUN npm ci
 
 COPY . .
 
+# Set API_BASE_URL at build time so Next.js inlines the correct value
+# (Next.js replaces process.env.X at build time via DefinePlugin)
+ARG API_BASE_URL=http://api-gateway:8000
+ENV API_BASE_URL=${API_BASE_URL}
+
 RUN npm run build
 
 # ===== Production =====