"Unauthorised for ECR" when using private image hosted on AWS ECR

pipelines:
  default:
    - step:
        name: Name of step
        image: 
          name: <aws_account_id>.dkr.ecr.<region>.amazonaws.com/namespace/php:latest
          aws: 
            access-key: $AWS_ACCESS_KEY
            secret-key: $AWS_SECRET_KEY
        services:
          - redis
        script:
          - pwd
          - ls -l */*

definitions:
  services:
    redis:
      image: redis:4.0