The Web application is built with Docker (docker-compose), and it is a way to access the API of the local environment.
I will explain with the assumption that you use Docker for Mac.
For example, if you start a local server and set the port as 3001, you can't access it from docker if you set the host as
You can access local server if you set the host as
From Docker for Mac ver17.06, the recommended DNS for lacalhost is
host.docker.internal is recommended instead of
docker.for.mac.localhost as DNS for localhost.