As-is, the nginx sample configuration maps the location from the
original URL (`/admin/`) to the location in the `pass_proxy` argument
`http://localhost:3000/` (`/`) .
Headplane does not have a route for `/`, so it returns a `404 Not Found`
with this configuration. Headplane _does_ have a route for `/admin/`. By
omitting the location portion of the `pass_proxy` argument, the location
from the original URL is used (c.f.
https://docs.nginx.com/nginx/admin-guide/web-server/reverse-proxy/#passing-a-request-to-a-proxied-server),
serving up the correct content.
Agatha V. Lovelace