NGINX 413 Request Entity Too Large

client_max_body_size をサーバーディレクティブに追加

http {
  include       mime.types;
  default_type  application/octet-stream;
  sendfile        on;
  keepalive_timeout  65;
  server {
    client_max_body_size 20M;
    listen       80;
    server_name  localhost;
    # Main location
    location / {
      proxy_pass http://127.0.0.1:8000/;  
    }
  }
}