I found a way to make it work. First connect to your Gitlab with command line and search for your Docker CONTAINER_ID

docker ps -all

eg.

docker exec -it d0bbe0e1e3db bash <-- with your given CONTAINER_ID

$ gitlab-rails console -e production

user = User.where(id: 1).first
user.password = 'your secret'
user.password_confirmation = 'your secret'
user.save
exit

I had a similar problem. Even if I set the environment variable, it seems that my gitlab ce still can't login. So I reset the password with the "root" user.

# dir: /etc/gitlab
gitlab-rake "gitlab:password:reset[root]"

Wait 10 minutes. And result

Enter password:
Confirm password:
Password successfully updated for user with username root.

Then all works as usual.

The solution is to set the GITLAB_OMNIBUS_CONFIG as an environment variable.

This works:

services:
  // other services..
  gitlab:
    image: 'gitlab/gitlab-ce'
    restart: always
    environment:
      GITLAB_OMNIBUS_CONFIG: |
        gitlab_rails['initial_root_password'] = 'adminadmin'
    ports:
      - '80:80'
      - '443:443'
      - '22:22'
      - "127.0.0.1:8081:80"
    volumes:
      - '/etc/gitlab'
      - '/var/log/gitlab'
      - '/var/opt/gitlab'
    networks:
      - backend

When starting up the service, this is confirmed:

gitlab_1       |               == Seed from /opt/gitlab/embedded/service/gitlab-rails/db/fixtures/production/002_admin.rb
gitlab_1       |               Administrator account created:
gitlab_1       |               
gitlab_1       |               login:    root
gitlab_1       |               password: adminadmin