We would like to use an external HTTPS load balancer for serving static error page in case application running on compute engine is down. We have multiple applications running on compute engine in form of Docker services, routing on CE is performed by reverse proxy (Traefik), also deployed as Traefik service. We have just one CE instance in unmanaged instance group. This is our architecture:

Let's say that photo gallery Docker service (https://gallery.company.com) is down. In this case, the HTTPS load balancer should behave as follows:

• blog.company.com should work normally
• gallery.company.com should show static error page
• shop.company.com should work normally

How can we achieve this per-URL health check (I am not interested in health of instance group as a whole) and redirection in case service is down?