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).
Create nginx-k8s.conf that proxies to localhost:8000 instead of
api-gateway:8000 for K8s pods where both containers share a network
namespace. Update Dockerfile.dashboard to accept a NGINX_CONF build arg
(defaults to docker/nginx.conf for docker-compose compatibility).