fix: hardcode nginx-k8s.conf in dashboard Dockerfile

The buildx plugin was not passing NGINX_CONF build arg correctly,
causing the docker-compose nginx config (with hostname api-gateway)
to be used instead of the K8s one (with localhost).

.woodpecker.yml

@@ -48,8 +48,6 @@ steps:
       repo: viktorbarzin/trading-bot-dashboard
       dockerfile: docker/Dockerfile.dashboard
       context: .
-      build_args:
-        - NGINX_CONF=docker/nginx-k8s.conf
       cache_from: viktorbarzin/trading-bot-dashboard:latest
       tags:
         - "${CI_PIPELINE_NUMBER}"