Kubernetes cluster domain defaults to 'cluster.local', but can also be customized.
We need to determine cluster domain to set up in-cluster forwarding to our egress proxies.
This was previously hardcoded to 'cluster.local', so was the egress proxies were not usable in clusters with custom domains.
This PR ensures that we attempt to determine the cluster domain by parsing /etc/resolv.conf.
In case the cluster domain cannot be determined from /etc/resolv.conf, we fall back to 'cluster.local'.
Updates tailscale/tailscale#10399,tailscale/tailscale#11445
Signed-off-by: Irbe Krumina <irbe@tailscale.com>
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