Problem with docker network and /etc/resolv.conf

First case:

I have the following docker-compose file

version: '3.4'

services:
  api:
    image: ${DOCKER_REGISTRY}project.api
    ports:
      - "5080:5080"
      - "5081:5081"
    build:
      context: .
      dockerfile: src/Api/Dockerfile.Debug
    environment:
      - ASPNETCORE_URLS=http://+:5080;http://+:5081;http://+:5443
      - ASPNETCORE_HTTP_PORT=5080
      - MANAGEMENT_HTTP_PORT=5081
      - ASPNETCORE_ENVIRONMENT=Local

When doing docker-compose up the container cannot access some endpoint that is supposed to give a token to the api running in the container. So I am having a timeout error.

Second case: However, with the following file it works with no problem.

version: '3.4'

services:
  api:
    image: ${DOCKER_REGISTRY}project.api
    build:
      context: .
      dockerfile: src/Api/Dockerfile.Debug
    environment:
      - ASPNETCORE_URLS=http://+:5080;http://+:5081;http://+:5443
      - ASPNETCORE_HTTP_PORT=5080
      - MANAGEMENT_HTTP_PORT=5081
      - ASPNETCORE_ENVIRONMENT=Local
    network_mode: host

It's worth noticing that on my host machine, the token-endpoint is accessible only when my vpn is running.

I've tried to to see what's is the /etc/resolv.conf/ in the containers in both cases (using docker exec -it "container_id" bash):

First Case: (the non-working case)

search XXX.biz YYY.biz
nameserver 127.0.0.11
options ndots:0

Second Case: (the working case)

search XXX.biz  YYY.biz
nameserver 10.10.***.***
nameserver 10.11.***.***
nameserver 8.8.8.8

I tried to add the missing entries nameserver 10.10.. nameserver 10.11.. nameserver 8.8.8.8

in daemon.json as following:

{
    "dns": [ "10.10.***.***", "10.11.***.***", "8.8.8.8" ],
    "dns-search": [ "XXX.biz", "YYY.biz" ]
}

but it doesn't solve the problem with the first case and I still don't see those entries inside the /etc/resolve.conf/ file inside my container.

Any ideas how to fix the first case?

PS: I have a debian 10 machine

I have tried all of the following attempts, nothing works

Attempt 1: I also tried to add dns entries in docker-compose file but it doesn't work neither.

Attempt 2: Adding dns entries in docker-compose and also networ_mode : brdige. This also adds the entries in the /etc/resolv.conf but the app still cannot get the token.

Attempt 3: Manual mounting where ./resolv.conf file contains what is expected.

volumes:
- ./resolv.conf:/etc/resolv.conf 

This also adds the entries in the /etc/resolv.conf but the app still cannot get the token.

The error is

An error has occurred in Timed KeyCloak Authentificated Service
api_1  |       System.Net.Http.HttpRequestException: Resource temporarily unavailable (oauth2.XXX.com:443)

The problem seems to be related to accessing the internet generally and not just accessing the token provider. Indeed, executing the following script within the container returns "Offline"

echo -e "GET http://google.com HTTP/1.0\n\n" | nc google.com 80 > /dev/null 2>&1

if [ $? -eq 0 ]; then
    echo "Online"
else
    echo "Offline"
fi
Offline