continuous integration - CircleCI - Docker Login and Private Env Vars

I am configuring CircleCI to automatically build and upload to Docker Hub multiple container of an open source library I contribute to.

Everything works fine except when I try to login on docker using the CLI.
I think that my builds have trouble to access private env vars, however I tried everything to fix it...

My configuration file is the following:

version: 2
jobs:
  build:
    working_directory: ~/build
    docker:
      - image: docker:git
    steps:
      - checkout
      - setup_remote_docker:
          reusable: true

          exclusive: true

      - run:
          name: Connect to Docker Hub
          command: |
            docker login -u ${DOCKER_USER} -p ${DOCKER_PASS}

I obtain the following error:

#!/bin/sh -eo pipefail
docker login -u $DOCKER_USER -p $DOCKER_PASS
"docker login" requires at most 1 argument.
See 'docker login --help'.

Usage:  docker login [OPTIONS] [SERVER] [flags]

Log in to a Docker registry
Exited with code 1

The variables are of course defined:

I would like to precise, that this runs on a branch of the main project and that I don't need to run it on project forks.

I'm usually quite use to Travis CI, and this is so much easier.

Answer

I have found the answer, this container is very sensitive about the way env vars are defined:

This command should be used:

docker login -u "$DOCKER_USER" -u "$DOCKER_PASS"

alternatively, the following can be used:

echo "$DOCKER_PASS" | docker login -u "$DOCKER_USER" --password-stdin