Let's say I want a docker-compose.yml that sets 2 services : a postgresql server and adminer (a kind of phpmyadmin). If I declare

# docker-compose.yml
version: '3.1'
services :
    myservername:
        image: postgres:10-alpine
        ports:
            - 5432:5432
        environment:
            POSTGRES_USER: user
            POSTGRES_PASSWORD: password
            POSTGRES_DB: mydb
    admin:
        image: adminer
        restart: always
        ports:
        - 8080:8080

With a docker-compose up -d and then on localhost:8080 I can connect to postgres :

That works like a charm. Perfect. But....

But due to specific configurations files in my project, I need that the postgres server name be localhost. So I tried :

# docker-compose.yml
version: '3.1'
services :
    localhost: # <----- that's the change
        image: postgres:10-alpine
        ports:
            - 5432:5432
        environment:
            POSTGRES_USER: user
            POSTGRES_PASSWORD: password
            POSTGRES_DB: mydb
    admin:
        image: adminer
        restart: always
        ports:
            - 8080:8080

And when I try to connect : SQLSTATE[08006] [7] could not connect to server: Connection refused Is the server running on host "localhost" (127.0.0.1) and accepting TCP/IP connections on port 5432? could not connect to server: Address not available Is the server running on host "localhost" (::1) and accepting TCP/IP connections on port 5432?

Any idea ?