Add boot script for Tailscale agent, running in a container

[rjackson90]

I thought it would be cool to run a Tailscale exit node on my UDM Pro, along with my PiHole. That way I can have ad-free and secure internet access wherever I happen to be. I really appreciate all these boot scripts, and wanted to contribute back. This is my first time trying to run anything custom on my Unifi hardware, so there's a pretty good chance I did something wrong! Feedback is appreciated.

I have uploaded the script to my own UDM Pro running the most recent available firmware, and it appears to work. I am submitting this PR while connected to the internet via my Tailscale exit node.