Document custom DNS port used by vcluster

[irizzant]

What happened?

Following up to #2465, I spent a huge amount of time trying to figure out why DNS was not working inside vcluster deployed in EKS and ended up discovering that a custom port (1053) is used for DNS traffic.

The official deployment guide on AWS doesn't document this and makes use of eksctl which creates allow-all AWS security groups, which makes everything work fine.

Whoever uses different tools (like Terraform) to create EKS clusters though is going to have hard times trying to figure out why by default vcluster DNS doesn't work unless the workloads are deployed in the same node as CoreDNS pod.

Users need to know that a custom DNS port is required to allow them configuring their AWS security groups accordingly.

What did you expect to happen?

DNS custom port 1053 usage should be documented

How can we reproduce it (as minimally and precisely as possible)?

Deploy an EKS cluster with Terraform, try to schedule a workload in a node different than CoreDNS pod and try to resolve DNS names

Anything else we need to know?

No response

Host cluster Kubernetes version

$ kubectl version
# paste output here

vcluster version

$ vcluster --version
# paste output here

VCluster Config

# My vcluster.yaml / values.yaml here