nginx keep port number when 301 redirecting

If nginx is not listening on port 8001, it cannot know which port to use in the redirect. You will need to specify it explicitly:

location ~ /foo(.*)$ {
    return 301 $scheme://$http_host$1;
}
location ~ /adminfoo(.*)$ {
    return 301 $scheme://$http_host/admin$1;
}

The $http_host variable consists of the hostname and port from the original request. See this document for details.