docker-compose.yml configuring GitLab with Container Registry, Pages, CI, Mattermost (enabled WebRTC), and some other options

docker-compose.yml

version: '2'
services:
  gitlab:
    container_name: gitlab
    image: gitlab/gitlab-ce:latest
    restart: always
    environment:
      GITLAB_OMNIBUS_CONFIG: |
        ## GitLab configuration settings
        ##! Check out the latest version of this file to know about the different
        ##! settings that can be configured by this file, which may be found at:
        ##! https://gitlab.com/gitlab-org/omnibus-gitlab/raw/master/files/gitlab-config-template/gitlab.rb.template

        ## GitLab URL
        ##! URL on which GitLab will be reachable.
        ##! For more details on configuring external_url see:
        ##! https://docs.gitlab.com/omnibus/settings/configuration.html#configuring-the-external-url-for-gitlab
        external_url 'http://gitlab.example.com'

        ################################################################################
        ## gitlab.yml configuration
        ##! Docs: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/settings/gitlab.yml.md
        ################################################################################

        gitlab_rails['gitlab_ssh_host'] = 'ssh.gitlab.example.com'
        gitlab_rails['time_zone'] = 'UTC'

        ### Email Settings
        # gitlab_rails['gitlab_email_enabled'] = true
        # gitlab_rails['gitlab_email_from'] = 'example@example.com'
        # gitlab_rails['gitlab_email_display_name'] = 'Example'
        # gitlab_rails['gitlab_email_reply_to'] = 'noreply@example.com'
        # gitlab_rails['gitlab_email_subject_suffix'] = ''

        ### Default project feature settings
        # gitlab_rails['gitlab_default_projects_features_issues'] = true
        # gitlab_rails['gitlab_default_projects_features_merge_requests'] = true
        # gitlab_rails['gitlab_default_projects_features_wiki'] = true
        # gitlab_rails['gitlab_default_projects_features_snippets'] = true
        # gitlab_rails['gitlab_default_projects_features_builds'] = true
        # gitlab_rails['gitlab_default_projects_features_container_registry'] = true

        ### Backup Settings
        ###! Docs: https://docs.gitlab.com/omnibus/settings/backups.html

        # gitlab_rails['backup_upload_connection'] = {
        #   'provider' => 'AWS',
        #   'region' => 'eu-west-1',
        #   'aws_access_key_id' => 'AKIAKIAKI',
        #   'aws_secret_access_key' => 'secret123'
        # }
        # gitlab_rails['backup_upload_remote_directory'] = 'my.s3.bucket'

        ### GitLab email server settings
        ###! Docs: https://docs.gitlab.com/omnibus/settings/smtp.html
        ###! **Use smtp instead of sendmail/postfix.**

        # gitlab_rails['smtp_enable'] = true
        # gitlab_rails['smtp_address'] = 'smtp.server'
        # gitlab_rails['smtp_port'] = 465
        # gitlab_rails['smtp_user_name'] = 'smtp user'
        # gitlab_rails['smtp_password'] = 'smtp password'
        # gitlab_rails['smtp_domain'] = 'example.com'
        # gitlab_rails['smtp_authentication'] = 'login'
        # gitlab_rails['smtp_enable_starttls_auto'] = true
        # gitlab_rails['smtp_tls'] = false

        ################################################################################
        ## GitLab Nginx
        ##! Docs: https://docs.gitlab.com/omnibus/settings/nginx.html
        ################################################################################

        # nginx['ssl_certificate'] = '/etc/gitlab/ssl/certificate.pem'
        # nginx['ssl_certificate_key'] = '/etc/gitlab/ssl/certificate.key'
        # nginx['listen_port'] = 80
        # nginx['listen_https'] = false

        ################################################################################
        ## Container Registry settings
        ##! Docs: https://docs.gitlab.com/ce/administration/container_registry.html
        ################################################################################

        registry_external_url 'http://registry.gitlab.example.com'

        ### Registry backend storage
        ###! Docs: https://docs.gitlab.com/ce/administration/container_registry.html#container-registry-storage-driver
        # registry['storage'] = {
        #   's3' => {
        #     'accesskey' => 'AKIAKIAKI',
        #     'secretkey' => 'secret123',
        #     'bucket' => 'gitlab-registry-bucket-AKIAKIAKI'
        #   }
        # }

        ### Registry notifications endpoints
        # registry['notifications'] = [
        #   {
        #     'name' => 'test_endpoint',
        #     'url' => 'https://gitlab.example.com/notify2',
        #     'timeout' => '500ms',
        #     'threshold' => 5,
        #     'backoff' => '1s',
        #     'headers' => {
        #       'Authorization' => ['AUTHORIZATION_EXAMPLE_TOKEN']
        #     }
        #   }
        # ]

        ################################################################################
        ## Registry NGINX
        ################################################################################

        # registry_nginx['ssl_certificate'] = '/etc/gitlab/ssl/certificate.pem'
        # registry_nginx['ssl_certificate_key'] = '/etc/gitlab/ssl/certificate.key'
        # registry_nginx['listen_port'] = 80
        # registry_nginx['listen_https'] = false

        ################################################################################
        ## GitLab Pages
        ##! Docs: https://docs.gitlab.com/ce/pages/administration.html
        ################################################################################

        ##! Define to enable GitLab Pages
        pages_external_url 'http://pages.example.com/'

        ################################################################################
        ## GitLab Pages NGINX
        ################################################################################

        # pages_nginx['ssl_certificate'] = '/etc/gitlab/ssl/certificate.pem'
        # pages_nginx['ssl_certificate_key'] = '/etc/gitlab/ssl/certificate.key'
        # pages_nginx['listen_port'] = 80
        # pages_nginx['listen_https'] = false

        ################################################################################
        ## GitLab Mattermost
        ##! Docs: https://docs.gitlab.com/omnibus/gitlab-mattermost
        ################################################################################

        mattermost_external_url 'http://mattermost.example.com'

        # mattermost['aws'] = {'S3AccessKeyId' => '123', 'S3SecretAccessKey' => '123', 'S3Bucket' => 'aa', 'S3Region' => 'bb'}

        # mattermost['email_enable_sign_up_with_email'] = true
        # mattermost['email_enable_sign_in_with_email'] = true
        # mattermost['email_enable_sign_in_with_username'] = false
        # mattermost['email_send_email_notifications'] = false
        # mattermost['email_require_email_verification'] = false
        # mattermost['email_smtp_username'] = nil
        # mattermost['email_smtp_password'] = nil
        # mattermost['email_smtp_server'] = nil
        # mattermost['email_smtp_port'] = nil

        # mattermost['file_amazon_s3_access_key_id'] = nil
        # mattermost['file_amazon_s3_bucket'] = nil
        # mattermost['file_amazon_s3_secret_access_key'] = nil
        # mattermost['file_amazon_s3_region'] = nil
        # mattermost['file_amazon_s3_endpoint'] = nil
        # mattermost['file_amazon_s3_bucket_endpoint'] = nil
        # mattermost['file_amazon_s3_location_constraint'] = false
        # mattermost['file_amazon_s3_lowercase_bucket'] = false
        # mattermost['file_amazon_s3_ssl'] = true

        # mattermost['privacy_show_email_address'] = true
        # mattermost['privacy_show_full_name'] = true

        # mattermost['localization_server_locale'] = 'en'
        # mattermost['localization_client_locale'] = 'en'
        # mattermost['localization_available_locales'] = 'en,es,fr,ja,pt-BR'

        mattermost['webrtc_enable'] = true
        mattermost['webrtc_gateway_websocket_url'] = 'ws://mattermost.example.com:8188'
        mattermost['webrtc_gateway_admin_url'] = 'http://mattermost.example.com:7088/admin'
        mattermost['webrtc_gateway_admin_secret'] = 'janusoverlord'
        # mattermost['webrtc_gateway_stun_uri'] = nil
        # mattermost['webrtc_gateway_turn_uri'] = nil
        # mattermost['webrtc_gateway_turn_username'] = nil
        # mattermost['webrtc_gateway_turn_shared_key'] = nil

        ################################################################################
        ## Mattermost NGINX
        ################################################################################

        # mattermost_nginx['ssl_certificate'] = '/etc/gitlab/ssl/certificate.pem'
        # mattermost_nginx['ssl_certificate_key'] = '/etc/gitlab/ssl/certificate.key'
        # mattermost_nginx['listen_port'] = 80
        # mattermost_nginx['listen_https'] = false

        ################################################################################
        ## Prometheus
        ##! Docs: https://docs.gitlab.com/ce/administration/monitoring/prometheus/
        ################################################################################

        # prometheus['enable'] = true
        # prometheus['monitor_kubernetes'] = true

        ################################################################################
        ## Prometheus Gitlab monitor
        ##! Docs: https://docs.gitlab.com/ce/administration/monitoring/prometheus/gitlab_monitor_exporter.html
        ################################################################################

        # gitlab_monitor['enable'] = true

        # To completely disable prometheus, and all of it's exporters, set to false
        # prometheus_monitoring['enable'] = true

        ################################################################################
        ## Gitaly
        ##! Docs:
        ################################################################################

        gitaly['enable'] = true
    volumes_from:
      - gitlab-data
    ports:
      - "80:80"
      - "2222:22"

  gitlab-data:
    image: busybox
    volumes:
      - /etc/gitlab
      - /var/log/gitlab
      - /var/opt/gitlab

  gitlab-runner:
    container_name: gitlab-runner
    image: gitlab/gitlab-runner:alpine
    restart: always
    volumes_from:
      - gitlab-runner-data
    volumes:
      - "/var/run/docker.sock:/var/run/docker.sock"
      - "$HOME/.docker/config.json:/root/.docker/config.json"

  gitlab-runner-data:
    image: busybox
    volumes:
      - /etc/gitlab-runner

  mattermost-webrtc:
    container_name: mattermost-webrtc
    image: mattermost/webrtc:latest
    restart: always
    ports:
      - "7088:7088"
      - "8188:8188"

Have you ever try to make it work with nginx-proxy and docker-letsencrypt-nginx-proxy-companion ?

@trash-anger No, I haven't. But I have made it work with AWS ELB and Certificate Manager.
For example, if you use https via proxy, configure like this

external_url 'https://gitlab.example.com'
nginx['listen_port'] = 80
nginx['listen_https'] = false