This change trims the mountPoint from the request URL path before
sending the request to the reverse proxy.
Today if you mount a proxy at `/foo` and request to
`/foo/bar/baz`, we leak the `mountPoint` `/foo` as part of the request
URL's path.
This fix makes removed the `mountPoint` prefix from the path so
proxied services receive requests as if they were running at the root
(`/`) path.
This could be an issue if the app generates URLs (in HTML or otherwise)
and assumes `/path`. In this case, those URLs will 404.
With that, I still think we should trim by default and not leak the
`mountPoint` (specific to Tailscale) into whatever app is hosted.
If it causes an issue with URL generation, I'd suggest looking at configuring
an app-specific path prefix or running Caddy as a more advanced
solution.
Fixes: #6571
Signed-off-by: Shayne Sweeney <shayne@tailscale.com>
c750186830