fix: nginx resolver for dynamic upstream DNS resolution

Use variable-based proxy_pass so nginx resolves the app hostname
at request time instead of startup time (DNS may not be ready at startup).

docker/frontend.Dockerfile

@@ -5,7 +5,8 @@ FROM nginx:alpine
 COPY dist /usr/share/nginx/html
 
 # nginx configuration for SPA
-RUN echo 'server { \
+RUN echo 'resolver 10.96.0.10 valid=10s ipv6=off; \
+server { \
     listen 80; \
     server_name _; \
     root /usr/share/nginx/html; \
@@ -14,12 +15,14 @@ RUN echo 'server { \
         try_files $uri $uri/ /index.html; \
     } \
     location /api/ { \
-        proxy_pass http://app:8080/api/; \
+        set $upstream_app http://app:8080; \
+        proxy_pass $upstream_app/api/; \
         proxy_set_header Host $host; \
         proxy_set_header X-Real-IP $remote_addr; \
     } \
     location /ws/ { \
-        proxy_pass http://app:8080/ws/; \
+        set $upstream_app http://app:8080; \
+        proxy_pass $upstream_app/ws/; \
         proxy_http_version 1.1; \
         proxy_set_header Upgrade $http_upgrade; \
         proxy_set_header Connection "upgrade"; \