Optional `ExtraHosts: { hostname: ip }` map per git entry. The
docker backend will surface these to the gate sidecar via
--add-host so the gate can resolve upstreams whose default
container DNS doesn't point at the reachable IP (e.g.
Tailscale-only hosts with a public DNS A record pointed
elsewhere). The agent-side insteadOf rewrite still keys off
the original hostname, so the manifest's Upstream URL stays
human-readable.
didericis
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