Add `with-known-host.go` source
Add `with-known-host.go` source

I think this stalled out when discovering that Linux namespaces didn't
play well with multi-threaded runtimes (e.g., Go), but the idea was to
mount a custom `~/.ssh/known_hosts` file in a mount NS in order to
override something for `ngrok.io` (where the "known" host fingerprint
changes based on both the IP address and which host is running `ngrok`).

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