Connection to registry at localhost:5000 is refused

I'm running a selfhosted gitlab instance via docker at gitlab.domain.com and I am using traefik as a reverse proxy, which also gives me the needed certificates (SSL). I just tried to enable the registry for my own build docker images at registry.domain.com. I can login via `docker login registry.domain.com. But clicking on 'Registry' at the gitlab frontend of any project gives me a 500 error.

So first I tried to look if I can access localhost:5000, but this is already failing:

$ docker exec -it gitlab /bin/bash

root@gitlab:/# curl -v http://localhost:5000
* Rebuilt URL to: http://localhost:5000/
*   Trying 127.0.0.1...
* TCP_NODELAY set
* connect to 127.0.0.1 port 5000 failed: Connection refused
*   Trying ::1...
* TCP_NODELAY set
* Immediate connect fail for ::1: Cannot assign requested address
*   Trying ::1...
* TCP_NODELAY set
* Immediate connect fail for ::1: Cannot assign requested address
* Failed to connect to localhost port 5000: Connection refused
* Closing connection 0
curl: (7) Failed to connect to localhost port 5000: Connection refused

Also:

root@gitlab:/# netstat -tanpu | grep -i listen
tcp        0      0 127.0.0.1:9093          0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.11:33383        0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:9100          0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:9229          0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:8080          0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:9168          0.0.0.0:*               LISTEN      -               
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      638/nginx       
tcp        0      0 127.0.0.1:8082          0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:9236          0.0.0.0:*               LISTEN      -               
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      21/sshd         
tcp        0      0 0.0.0.0:8060            0.0.0.0:*               LISTEN      638/nginx       
tcp        0      0 127.0.0.1:9121          0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:9090          0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:9187          0.0.0.0:*               LISTEN      -               
tcp6       0      0 :::9094                 :::*                    LISTEN      -               
tcp6       0      0 :::22                   :::*                    LISTEN      21/sshd 

I build the gitlab container by using docker-compose:

version: '3.3'

services:
  gitlab:
    image: gitlab/gitlab-ce:latest
    container_name: gitlab
    environment:
      GITLAB_OMNIBUS_CONFIG: |
        external_url = 'https://gitlab.domain.com'
        registry_external_url = 'https://registry.domain.com'
        gitlab_rails['registry_enabled'] = true
    ports:
      - '2222:22'
      - '5000:5000'
    volumes:
      - /opt/gitlab/config:/etc/gitlab
      - /opt/gitlab/logs:/var/log/gitlab
      - /opt/gitlab/data:/var/opt/gitlab
    networks:
      - proxy
    labels:
      - traefik.enable=true
      - traefik.ci.frontend.rule=Host:gitlab.domain.com
      - traefik.ci.port=80
      - traefik.reg.frontend.rule=Host:registry.domain.com
      - traefik.reg.port=80
      - traefik.docker.network=proxy

networks:
  proxy:
    external: true

My traefik configuration:

defaultEntryPoints = ["https","http"]

[entryPoints]
  [entryPoints.http]
  address = ":80"
    [entryPoints.http.redirect]
    entryPoint = "https"
  [entryPoints.https]
  address = ":443"
  [entryPoints.https.tls]

[docker]
domain = "domain.com"
watch = true
network = "proxy"

[acme]
email = "notifications@domain.com"
storage = "acme.json"
entryPoint = "https"
OnHostRule = true
  [acme.httpChallenge]
  entryPoint = "http"