Heroku + nginx-buildpack - nginx

Question

I am trying to host the TensorBoard on a Heroku instance, and to secure it, I have added nginx using the Nginx-Buildpack in front of it.
The idea is that Tensorboard will create the app on port 6006, and Nginx will redirect this port to the external port provided by Heroku $Port.
When I start the app, I have the following error:
TensorBoard attempted to bind to port 6006, but it was already in use
My config files are as follows:
Procfile
web: bin/start-nginx tensorboard --logdir="/app/" --host=http://127.0.0.1 --port=6006
config/nginx.conf.erb
daemon off;
#Heroku dynos have at least 4 cores.
worker_processes <%= ENV['NGINX_WORKERS'] || 4 %>;
events {
use epoll;
accept_mutex on;
worker_connections 1024;
}
http {
gzip on;
gzip_comp_level 2;
gzip_min_length 512;
server_tokens off;
log_format l2met 'measure#nginx.service=$request_time
request_id=$http_x_request_id';
access_log logs/nginx/access.log l2met;
error_log logs/nginx/error.log;
include mime.types;
default_type application/octet-stream;
sendfile on;
#Must read the body in 5 seconds.
client_body_timeout 5;
#upstream app_server {
# server unix:/tmp/nginx.socket fail_timeout=0;
#}
server {
listen <%= ENV["PORT"] %>;
server_name http://127.0.0.1;
keepalive_timeout 5;
root /app;
port_in_redirect off;
#index index.html index.htm;
location = / {
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_redirect off;
proxy_pass http://127.0.0.1:6006;
}
}
}