fix: connect frontend to sync server for multiplayer

NEXT_PUBLIC_SYNC_URL was never set, so useRoom always fell into
local-only mode — users couldn't see other joiners. Added build arg
to bake wss://sync.rmaps.online into the client bundle.

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

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
 

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: