build: add Docker Compose setup for Dokploy deployment

- Multi-stage Dockerfile for Go backend (golang:1.25-alpine -> alpine:3)
- Multi-stage Dockerfile for Next.js frontend (bun:1 -> node:22-alpine)
- docker-compose.yml with backend + frontend services, health checks
- Next.js standalone output + API rewrites to proxy /api/* to backend
- .dockerignore files for both services
- .env.example documenting required environment variables

frontend/.dockerignore

@@ -0,0 +1,9 @@
+node_modules/
+.next/
+out/
+build/
+.git
+.gitignore
+Dockerfile
+.dockerignore
+.env*