Add flag configuration Via Env Variables

[h4ux]

Suggestion:
I think that it's a good idea to support tailscale client flags via env vars.
This will allow the use of the node as an exit-node for example.

[willnorris]

we've exposed the new-ish web UI in golink to allow managing the Tailscale part of it remotely. Instructions for how to enable that on your tailnet can be found here. Once you've done that, you can go to http://go:5252/, login (again), and then manage several aspects of the device including making it an exit node. We're working on adding more stuff to that web UI over time.

There are still other valid reasons to potentially expose some setting via environment variables, but I suspect we'd probably add support for the newer JSON config before that, which is the direction some of our other containers are moving in (see #10993)