fix(nginx): add Docker DNS resolver for Swarm/dynamic service discovery

- Add resolver 127.0.0.11 directive for Docker's internal DNS
- Use variable-based proxy_pass to force per-request DNS resolution
- Fix 502 Bad Gateway error on root path in Docker Swarm deployments

The issue was that nginx caches DNS lookups at startup, but in Docker
Swarm where service IPs can change dynamically, this caused stale DNS
entries leading to 502 errors for proxied requests.

Bumps version to 2.2.3

backend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "picpeak-backend",
-  "version": "2.2.2",
+  "version": "2.2.3",
   "description": "Backend for PicPeak event photo sharing platform",
   "main": "server.js",
   "scripts": {