If you are debugging an application inside a docker container and want to connect to the SQL server running on your localhost:3000, you cannot give the localhost:3000 as a connection string. Docker doesn't know how to connect to your local host. To solve that, you can follow the below methods.

If you are a Mac or Windows user, you could simply replace the localhost:3000 with **host.docker.internal:3000** inside your container.

If you are a Linux user, you could add a **--add-host**flag for the docker run. Start your containers with this flag to expose the host string:

docker run --add-host host.docker.internal:host-gateway containername:latest

The --add-host flag adds an entry to the container’s /etc/hosts file. The value shown above maps host.docker.internal to the container’s host gateway, which matches the real localhost value. You could replace host.docker.internal with your string if you prefer.

To get the same behaviour in docker-compose, you need to specify the host.docker.internal:host-gateway using the extra_hosts parameter.

version: "3.8"
services:

  ubuntu:
    image: ubuntu
    container_name: ubuntu
    extra_hosts:
      - "host.docker.internal:host-gateway"
    command: sleep infinity

Note: host.docker.internal which resolves to the internal IP address used by the host. This is for development purposes and will not work in a production environment outside of Docker Desktop for Windows.