Nginx simple proxy doesn't work in Docker

I've a backend and a frontend that run in two containers in Docker, and both are accessible from my browser with their ports:

localhost:8081 # frontend
localhost:8082 # backend

Now, I want to use an Nginx server that receives all the traffic (port 80) and redirects:

localhost -> to frontend
localhost/api -> to the backend

After trying almost everything in nginx.conf file (nothing worked), I found in an SO Question that the file I must modify is:

/etc/nginx/sites-enabled/default

So I try to modify it, and now nginx runs. And with runs, I mean that at least when I access localhost, nginx welcome page is shown. However, I still can't make my nginx route traffic (proxy way, not redirect).

Right now my docker-compose file has the following look (snippet for nginx plus those two services):

  nginx:
    image: nginx:latest
    ports:
      - 80:80
    volumes:
      - ./Dockerfiles/nginx/default:/etc/nginx/sites-enabled/default
    links:
      - frontend
      - backend
    depends_on:
      - frontend
      - backend
  frontend:
    build: ./Dockerfiles/frontend
    volumes:
      - ./Dockerfiles/frontend/www/src:/usr/src/app
      - ./logs/frontend/httpd:/var/logs/httpd
    ports:
      - "8081:3000"
    links:
      - backend
  backend:
    build: ./Dockerfiles/backend
    volumes:
      - ./Dockerfiles/backend/www:/var/www/html
      - ./logs/backend/httpd:/var/logs/httpd
    ports:
      - "8082:80"
    links:
      - "database"
      - "redis"
    depends_on:
      - database
      - redis

My default file as said before, for nginx config is:

server {
        listen 80 default_server;

        # Add index.php to the list if you are using PHP
        index index.html index.htm index.php index.nginx-debian.html;

        server_name _;

        location / {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                #try_files $uri $uri/ =404;
            proxy_pass http://frontend:8081/;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection 'upgrade';
            proxy_set_header Host $host;
            proxy_cache_bypass $http_upgrade;
        }

        location /api {
            proxy_pass http://backend:8082/public/;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection 'upgrade';
            proxy_set_header Host $host;
            proxy_cache_bypass $http_upgrade;
        }

}

And when I try to access my /api url for example, the error that nginx is telling my is:

nginx_1 | 2018/12/17 21:44:09 [error] 6#6: *1 open() "/usr/share/nginx/html/api" failed (2: No such file or directory), client: 172.20.0.1, server: localhost, request: "GET /api HTTP/1.1", host: "localhost"

Which seems like it's trying to retrieve a file from local filesystem (something that I can't understand, given the conf. files).

Any lead, idea or tip?