title: postgres with docker compose gives FATAL: role "root" does not exist error
body: <p>I'm trying to create a simple demo with postgres on a local windows machine with docker desktop.<br>
  This is my yaml docker compose file named <code>img.yaml</code>:</p>
  
  <pre><code>version: '3.6'
  
  services:
  
      postgres-demo:
        image: postgres:11.5-alpine
        container_name: postgres-demo
        environment:
          - POSTGRES_USER=postgres
          - POSTGRES_PASSWORD=Welcome
          - POSTGRES_DB=conference_app
        healthcheck:
          test: ["CMD-SHELL", "pg_isready -U postgres"]
          interval: 10s
          timeout: 5s
          retries: 5
        ports:
          - 5432:5432
        volumes:
          - .:/var/lib/my_data
        restart: always
  
  </code></pre>
  
  <p>I'm running it using the command:
  <code>docker-compose -f img.yaml up</code>
  And get the following output:</p>
  
  <pre><code>Starting postgres-demo ... done
  Attaching to postgres-demo
  postgres-demo    | 2020-02-12 17:07:46.487 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
  postgres-demo    | 2020-02-12 17:07:46.487 UTC [1] LOG:  listening on IPv6 address "::", port 5432
  postgres-demo    | 2020-02-12 17:07:46.508 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
  postgres-demo    | 2020-02-12 17:07:46.543 UTC [18] LOG:  database system was shut down at 2020-02-12 17:07:10 UTC
  postgres-demo    | 2020-02-12 17:07:46.556 UTC [1] LOG:  database system is ready to accept connections
  </code></pre>
  
  <p>And then, opening bash into the container with the command:
  <code>docker exec -it d47056217a97 bash</code><br>
  I want to watch the databases in container so I run in the bash the command:
  <code>psql \dt</code><br>
  And get the error:
  <code>psql: FATAL:  role "root" does not exist</code>.<br>
  Trying to create the database using the command: <code>psql&gt; create database conference_app;</code> gives the error: <code>psql: FATAL:  role "conference_app" does not exist</code>.<br>
  I'm puzzled. What am I doing wrong? Is my yaml missing something?</p>