How do I access a server on localhost with nginx docker container?

If you’re using docker-for-mac 18.03 or newer it auto creates a special DNS entry host.docker.internal that dynamically binds to the host inet ip. You can then use the dns name to proxy services running on the host machine from inside a container as a stand-in for localhost.

i.e. an nginx config file:

server {
  listen 0.0.0.0:80;
  server_name localhost;

  location / {
    proxy_pass http://host.docker.internal:3000;
  }
}