diff --git a/frontend/Dockerfile b/frontend/Dockerfile
index ed39f80..a5cbc29 100644
--- a/frontend/Dockerfile
+++ b/frontend/Dockerfile
@@ -24,6 +24,12 @@ RUN npm ci
 COPY . .
 
 ENV ASTRO_TELEMETRY_DISABLED=1
+
+# PUBLIC_API_URL must be set at build time for Vite to inline it.
+# Empty string = same-origin (Caddy routes /api/* to backend).
+ARG PUBLIC_API_URL=""
+ENV PUBLIC_API_URL=${PUBLIC_API_URL}
+
 RUN npm run build