No need to override the default headers. In the docker-compose.yml file in the GITLAB_OMNIBUS_CONFIG section just add nginx['listen_https'] = false:

environment:
  GITLAB_OMNIBUS_CONFIG: |
    external_url 'https://gitlab.mydomain.de'
    letsencrypt['enabled'] = false
    nginx['listen_https'] = false
    nginx['listen_port'] = 80

Hthis will disable SSL inside docker and your reverse_proxy can do the SSL-thing

No need to override the default headers. In the docker-compose.yml file in the GITLAB_OMNIBUS_CONFIG section just add nginx['listen_https'] = false:

environment:
  GITLAB_OMNIBUS_CONFIG: |
    external_url 'https://gitlab.mydomain.de'
    letsencrypt['enabled'] = false
    nginx['listen_https'] = false
    nginx['listen_port'] = 80

This will disable SSL inside docker and your reverse_proxy can do the SSL-thing

see https://docs.gitlab.com/omnibus/settings/nginx.html#external-proxy-and-load-balancer-ssl-termination