diff --git a/Dockerfile b/Dockerfile
index 52acc98..1b0d4ca 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -23,6 +23,9 @@ COPY rmaps-online/ .
 
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NODE_ENV=production
+# Sync server URL baked into client bundle at build time
+ARG NEXT_PUBLIC_SYNC_URL
+ENV NEXT_PUBLIC_SYNC_URL=${NEXT_PUBLIC_SYNC_URL}
 
 RUN npm run build
 
diff --git a/docker-compose.yml b/docker-compose.yml
index 6c26880..c0868f1 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -5,6 +5,8 @@ services:
     build:
       context: ..
       dockerfile: rmaps-online/Dockerfile
+      args:
+        - NEXT_PUBLIC_SYNC_URL=wss://sync.rmaps.online
     container_name: rmaps-online
     restart: unless-stopped
     environment: