fix: resolve container startup issues

backend/Dockerfile:
- Add ENV PYTHONPATH=/app so alembic can import app.config
- Change CMD to bash entrypoint.sh (volume mount breaks chmod +x)

nginx/nginx.conf:
- Add resolver 127.0.0.11 (Docker internal DNS)
- Use set $backend/$frontend variables so nginx resolves
  hostnames per-request instead of at startup

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

nginx/nginx.conf

@@ -1,16 +1,21 @@
 server {
     listen 80;
 
+    # Docker внутренний DNS — резолвим хосты в момент запроса, а не при старте nginx
+    resolver 127.0.0.11 valid=30s;
+
     # API запросы → FastAPI backend
     location /api/ {
-        proxy_pass http://backend:8000;
+        set $backend http://backend:8000;
+        proxy_pass $backend;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
     }
 
     # Всё остальное → Vite dev server (с поддержкой WebSocket для HMR)
     location / {
-        proxy_pass http://frontend:5173;
+        set $frontend http://frontend:5173;
+        proxy_pass $frontend;
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection "upgrade";