Issue connecting to my postgres database instance

I am trying to connect to my postgres database via Docker. I have confirmed that it is running (port 5432) along with my database manager tool called Adminer (port 8080). So far so good!

Next, I attempt to make my database migrations. (Here's the Makefile) by running the following command:

FYI, migrate is a database migration tool for Golang

migrate -source file://migrations -database postgres://postgres:secret@localhost:5432/syndicate?sslmode=disable up

My database URL checks out: dbdriver://username:password@host:port/dbname?param1=true&param2=false

But receive this error:

error: pq: role "postgres" does not exist

Here's what I know:

• Database name is syndicate (database.env)
• Default database user is postgres (database.env)
• Password is secret (database.env)

Why would the default postgres user require a role?

I created this issue and have been narrating it as I receive feedback from various platforms. Here's my full repo

One last thing...

The only thing i could find that was similar to my issue is this documentation, PostgreSQL 1.3. Creating a Database stating:

Another response could be this:

createdb: could not connect to database postgres: FATAL: role "joe" does not exist

where your own login name is mentioned. This will happen if the administrator has not created a PostgreSQL user account for you. (PostgreSQL user accounts are distinct from operating system user accounts.) If you are the administrator, see Chapter 18 for help creating accounts. You will need to become the operating system user under which PostgreSQL was installed (usually postgres) to create the first user account

That is all, thank you!

Edit: As suggested, I added a POSTGRES_USER to database.env:

POSTGRES_PASSWORD=secret
POSTGRES_DB=syndicate
POSTGRES_USER=user

database.env is referred to in docker-compose.yml:

version: '3.1'
services:
  adminer:
    image: adminer
    restart: always
    ports:
      - 8080:8080
  db:
    image: "postgres"
    env_file:
      - database.env # configure postgres
    ports:
      - 5432:5432

Now receiving error: error: pq: role "user" does not exist