try to fully dynamically set what I think should be right

ansible/roles/pre-configure/templates/registries.yaml

@@ -3,10 +3,10 @@ mirrors:
   docker.io:
     endpoint:
       # Hardcoded, should be fixed before merging
-      - "{{ docker_mirror.server }}"
+      - "{{ docker_mirror.endpoint }}"
 configs:
   # TODO: Hardcoded because GitLab does not give us the right path
-  "open.greenhost.net":
+  "{{ docker_mirror.server }}":
     auth:
       username: "{{ docker_mirror.username }}"
       password: "{{ docker_mirror.password }}"